Calcium

Calcium (Ca) · also called Elemental calcium, calcio

Calcium is the most plentiful mineral in your body, and almost all of it is stored in bones and teeth. Dairy foods, fortified drinks, tofu, canned fish with bones and some leafy greens are the main food sources. In trials, calcium pills made little or no difference to fractures and gave only a small rise in bone density. Pills also went with more kidney stones and, in pooled trials, more heart events. The main cautions are kidney stones, too much calcium with vitamin D, and medicine interactions.

General information, not medical advice. Last substantive update October 26, 2026 · Not medically reviewed. A named clinician has not signed off on this page. How this page is made

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About Calcium

Calcium is a mineral your body needs for bones and teeth, muscles, nerves and blood clotting. Food is the main source. Calcium pills are meant for people who do not get enough from food, but the trials we read, mostly in people who were not known to be short, found little to no effect on fractures and a small rise in bone density. Pills went with more kidney stones and, in pooled trials, more heart events. Calcium can also lower how well some medicines and iron are absorbed.

What it is

Calcium is a mineral. MedlinePlus says you have more calcium than any other mineral, and almost all of it is stored in your bones and teeth. It also helps muscles move, nerves carry messages, blood vessels move blood and hormones be released. The NHS adds that it helps keep a normal heartbeat and lets blood clot normally.

Not getting enough calcium over a long time can lead to rickets in children and to osteomalacia or osteoporosis later in life (NHS, MedlinePlus). This page covers calcium as a nutrient and a supplement ingredient.

Supplements usually come as calcium carbonate or calcium citrate. MedlinePlus says carbonate is absorbed best with food and citrate with or without food, and that people with low stomach acid absorb citrate more easily. Calcium carbonate is also in some antacids. We read no trial comparing forms head to head.

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Food sources

Dairy foods such as milk, cheese and yogurt are the main sources for most people in the US (MedlinePlus). Other sources include kale, broccoli and bok choy, canned salmon and sardines eaten with their soft bones, almonds and tahini, and calcium-enriched cereals, juices, soy drinks and tofu.

Rough amounts per portion from MedlinePlus: 8 oz (240 mL) of nonfat milk or of calcium-fortified soy milk about 300 mg; 1.5 oz (45 g) of cheese about 300 mg; 6 oz (170 g) of yogurt about 300 mg; 3 oz (85 g) of sardines with bones about 325 mg; 1/2 cup of cooked turnip greens about 100 mg; 1/4 cup of almonds about 100 mg; 1 cup of shredded bok choy about 74 mg. Our tofu page lists 434 mg in half a cup (124 g) of regular tofu set with calcium sulfate (USDA values carried over from that page).

Wheat bran and foods with oxalic acid, such as spinach and rhubarb, bind calcium and limit absorption. The NHS says the same about spinach. Food labels show how much calcium a fortified product has. These numbers describe the foods and are not advice on how much to eat.

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What the research shows

Fractures: a 2026 BMJ review of 69 trials (153,902 adults, mostly community-dwelling and not at high risk) found calcium alone made little to no difference to any fracture (RR 0.91, 0.81 to 1.01; moderate certainty). Calcium with vitamin D gave RR 0.91 (0.84 to 0.99; high certainty). A 2015 review found signs of bias in the published supplement trials and no effect in the four lowest-bias trials.

Bone density: calcium from food or pills raised density by about 0.6 to 1.8% in adults over 50. The gain did not keep growing and the reviewers think it is unlikely to reduce fractures meaningfully.

Heart: pooled trials of calcium pills found about 15% more cardiovascular events in healthy postmenopausal women (RR 1.15, 1.06 to 1.25). Calcium from food showed no increase in cohort studies. This is debated, and we did not read analyses arguing the other way.

Kidney stones: calcium carbonate with vitamin D3 went with more stones in the WHI trial (HR 1.17), while calcium from food went with fewer stones in a cohort of men.

Pregnancy and PMS: a 2025 Cochrane review found calcium made little to no difference to pre-eclampsia in the large trials. One 1998 trial found lower PMS scores with calcium carbonate than with placebo.

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Food versus pills

The findings differ by source. Calcium eaten in food showed no increase in heart risk and went with fewer kidney stones in cohort studies. Pills went with more stones in a trial and more heart events in pooled trials. For bone density, food and pills gave similar small gains. Foods also bring other nutrients, such as protein and, in fortified milk, vitamin D. Cohort studies cannot prove cause, so the difference may partly reflect who takes pills.

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At a glance

Traditional uses history, not proof

Calcium is a nutrient the body needs, not a traditional remedy
Long use of calcium-rich foods is not proof that extra calcium pills treat any condition

Studied for research

Fractures, mixed certainty (calcium alone: little to no effect; with vitamin D: small effect)
Bone density, moderate certainty (a small rise that does not keep growing)
Heart disease, mixed certainty (pills linked to more events in pooled trials; food not)
Kidney stones, mixed certainty (food linked to fewer, pills with vitamin D to more)
Pre-eclampsia in pregnancy, evidence against a clear benefit (large trials: little to no difference)
Premenstrual symptoms, low certainty (one 1998 trial)

Main compounds

Calcium carbonate (supplement form, also in some antacids)
Calcium citrate (supplement form)
Calcium in food (dairy, fortified foods, tofu, canned fish with bones)

Common forms

Calcium carbonate or citrate pills, often with vitamin D (used in most trials)
Calcium from food and fortified drinks (used in some trials and cohort studies)
Multivitamins, antacids and other forms (lactate, gluconate, phosphate) were not covered by trials we read

Research links: 2026 BMJ review: calcium, vitamin D and fractures (69 trials) · 2015 BMJ review: calcium intake and bone density · 2021 meta-analysis: calcium pills and cardiovascular disease · Research Library · Genes & Biomarkers Library

What it is and its history

Accepted scientific name: Calcium (element Ca). A mineral nutrient, not a plant or an animal.
Common names: Calcium (English); Elemental calcium (English (label term for the calcium part of a supplement)); Calcio (Spanish)
Family or group: Major mineral
Harvest and processing: Not covered. The sources read do not describe how calcium supplements are made.
Look-alikes and adulterants: Name: Calcium carbonate; Note: A supplement form; also in some antacids. MedlinePlus says it is absorbed best with food.; Name: Calcium citrate; Note: A supplement form. MedlinePlus says it is absorbed well with or without food and more easily than carbonate by people with low stomach acid.; Name: Calcium lactate, gluconate and phosphate; Note: Other forms named by MedlinePlus; no trial of them was read.; Name: Magnesium; Note: Another major mineral with its own page; milk is also a source of phosphorus and magnesium, which MedlinePlus says help the body absorb and use calcium.

What is inside, and how it may work

What it contains and how those compounds act in cells and animals. Laboratory and animal mechanisms are not proven effects in people.

Main compounds

Compound	Family	Where it is concentrated	Activity studied	Level of evidence
Calcium carbonate	mineral salt	MedlinePlus: antacid chews or pills usually give 200 to 400 mg of calcium each; check the label	A common supplement form. Studied in the WHI trial, the PMS trial and the levothyroxine study. Absorbed best with food (MedlinePlus). ^{17 7 12 10}	Human data
Calcium citrate	mineral salt	Not stated in the sources read	A common supplement form, absorbed with or without food (MedlinePlus). Used to top up calcium intake in the hypercalciuria trial. ^{16 5}	Human data
Calcium in food	mineral in food	MedlinePlus portion amounts: 8 oz (240 mL) nonfat milk about 300 mg; 8 oz calcium-fortified soy milk about 300 mg; 1.5 oz (45 g) cheese about 300 mg; 6 oz (170 g) yogurt about 300 mg; 3 oz (85 g) sardines with bones about 325 mg; 1/2 cup cooked turnip greens about 100 mg; 1/4 cup almonds about 100 mg; 1 cup shredded bok choy about 74 mg. Our tofu page lists 434 mg in 1/2 cup (124 g) of regular tofu set with calcium sulfate (USDA, summary-only).	The main source for most people in the US is dairy. MedlinePlus says spinach, rhubarb and wheat bran bind calcium and limit absorption, and the NHS says the same about spinach. ^{17 19 23}	Human data

Mechanisms, receptors and pathways

Target or pathway	Gene or receptor	What was found	Level
Bone and teeth mineral	Not specified in the sources read	MedlinePlus says the body stores almost all its calcium in bones and teeth, and that calcium also helps muscles move, nerves carry messages, blood vessels move blood and hormones be released. The NHS adds the heartbeat and normal blood clotting. ^{16 19}	Human data
Iron absorption in the gut	Not specified	In 126 volunteers, calcium reduced absorption of both non-heme and heme iron, which the authors take to mean an effect on iron transfer in the gut lining. ⁶	Human data
Levothyroxine binding	Not specified	A lab test showed levothyroxine adsorbs (sticks) to calcium carbonate at acid pH, and 20 patients taking both had lower free T4 and higher TSH. ¹⁰	Human data
Why pills might affect the heart	Not specified	Not established in the abstracts read. No mechanism was tested. ⁹	Hypothesis

Absorption, metabolism and elimination

Absorption: MedlinePlus says carbonate is absorbed best with food and citrate with or without food; vitamin D is needed to help the body absorb calcium; and absorption is better in smaller portions. Postmenopausal women absorb and retain less calcium.
Distribution: Almost all is stored in bones and teeth; the rest is in nerve cells, tissues, blood and other body fluids (MedlinePlus).
Elimination: High urine calcium occurred in 30.6% of older women on calcium and vitamin D in one trial.
Bioavailability: Calcium from dairy is a form the body easily absorbs (MedlinePlus). Plant foods high in oxalate or fibre give less usable calcium. Fortified foods and pills were not compared head to head in the sources read.

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Isolated compounds versus whole extracts

Calcium from food and calcium from pills behave differently in the studies read. In cohort studies, calcium from food showed no increase in heart risk and went with fewer kidney stones, while pills went with more stones and (in pooled trials) more heart events. For bone density, food and pills gave similar small gains in a 2015 review. Foods also carry other nutrients (for example protein, vitamin D in fortified milk) that pills lack.

Nutritional composition

Food amounts come from MedlinePlus, 'Calcium in diet' (review date 1/21/2025), read 2026-10-26. The tofu value is from our tofu page, which uses a USDA record that was not re-opened for this page. The USDA public key was rate-limited on 2026-10-26. These are descriptions of food content, not advice on how much to eat. Reference intakes differ by country and age; they are on the MedlinePlus page and are not repeated here.

6benefits and uses looked at

23studies and sources checked

Moderate certaintybest evidence rating found

5lab markers tracked in trials

1 moderate certainty3 mixed findings1 low certainty1 evidence against

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What was looked at

The 6 benefits and uses checked for Calcium, each with how strong the evidence is. Tap one to open it.

Bone mineral density (bone strength measure)Moderate certainty
Preventing fracturesMixed findings
Heart disease (a possible harm from pills, not a benefit)Mixed findings
Kidney stonesMixed findings
Pre-eclampsia in pregnancyEvidence against
Premenstrual symptoms (PMS)Low certainty

Lab markers tracked in the studies

Bone mineral density (BMD)Any fracture (health outcome)Coronary artery calcification (CAC)Premenstrual symptom score (17 items)Urinary tract stone (health outcome)

What is Calcium good for?

The main studied uses and how strong the human evidence is for each. Most trials tested calcium pills (often with vitamin D) in older and postmenopausal women, and results may not carry over to children, men or people who eat very little calcium. Safety findings from the same trials are on the Safety tab.

Preventing fracturesMixed findingsAcross 69 trials, calcium alone made little to no difference to any fracture (RR 0.91, 0.81 to 1.01; moderate certainty). Calcium with vitamin D lowered the risk slightly (RR 0.91, 0.84 to 0.99; high certainty). A 2015 review found calcium from food was not linked to fractures in most cohort studies. Only one trial in frail older women in care with low calcium and vitamin D intake showed a clear benefit. Bone mineral densityModerate certaintyIn adults over 50, calcium pills raised bone density by 0.7 to 1.8% and calcium from food by 0.6 to 1.8%. The gain did not keep growing and the reviewers think it is unlikely to cut fractures meaningfully. In the WHI trial, hip density was 1.06% higher than with placebo after about 7 years. Heart disease (a possible harm from pills)Mixed findingsA 2021 meta-analysis of 13 placebo-controlled trials found calcium pills raised cardiovascular risk (RR 1.15, 1.06 to 1.25) in healthy postmenopausal women. A 2020 analysis found calcium eaten in food did not change risk (RR 0.96) while pills raised heart attack risk (RR 1.14). A 10-year study linked supplement use with new artery calcification (RR 1.22). The question is debated and heart events were not the main aim of the trials. Kidney stonesMixed findingsCalcium from food went with fewer stones in 45,619 men (RR 0.66 adjusted). Calcium carbonate with vitamin D3 went with more stones in 36,282 postmenopausal women (HR 1.17, 1.02 to 1.34). Opposite directions: food was observational, the pills were a randomised trial. Pre-eclampsia in pregnancyEvidence againstA 2025 Cochrane review of 10 trials (37,504 women) found calcium may make little to no difference to pre-eclampsia (RR 0.83, 0.67 to 1.04, low certainty). Leaving out small studies: RR 0.92 (0.79 to 1.05), high certainty of little to no difference. Premenstrual symptomsLow certaintyIn one 1998 trial of 466 women, symptom scores fell 48% with calcium carbonate and 30% with placebo by the third cycle. One trial, self-rated symptoms; repeat trials were not read.

Benefits, one by one

Each card is a research question, not a promise. Tap a goal to filter. Open a card to see the studies, doses and limits.

Bone mineral density (bone strength measure)

Moderate certainty

In trials of adults over 50, calcium from food or from pills raised bone density by a small amount, about 0.6 to 1.8%. The gain did not keep growing over time. The reviewers think it is unlikely to cut fractures by a meaningful amount.

2 studiesStudied as calcium carbonate or citrate pills (often with vitamin D) …

Studies, doses and limits

What the trials found

A 2015 BMJ review pooled randomised trials in people over 50: 15 trials of calcium from food (1,533 people) and 51 trials of calcium pills (12,257 people). The WHI trial of 36,282 postmenopausal women (calcium carbonate with vitamin D3) found hip bone density 1.06% higher than placebo after about 7 years.

Results

A small, steady rise in bone density. It does not mean fewer fractures.

Effect sizes as reported

Pills: +0.7 to 1.8% at all five bone sites. Food sources: +0.6 to 1.0% at the total hip and whole body at one year, +0.7 to 1.8% at two years, no change at the forearm. WHI: hip density +1.06% (P below 0.01).

Why this rating

Moderate. Pooled randomised trials agree on a small rise, but bone density is a stand-in measure and the reviewers call the rise too small to matter for fractures.

Form and dose studied

Studied as calcium carbonate or citrate pills (often with vitamin D) and as dairy or fortified foods. Food and pills gave similar gains, except at the forearm.

Who was studied

Adults over 50, mostly postmenopausal women.

Limits

Stand-in measure, not fractures. Baseline diet did not change the size of the gain in the review. Results do not carry over to children, younger adults or men.

Why researchers looked

Almost all the body's calcium is stored in bones and teeth, so researchers tested whether extra calcium adds to bone density.

Traditional use

Calcium is a nutrient, not a traditional remedy. Long use of calcium-rich foods is not proof that extra calcium pills treat any condition.

Proposed mechanism Hypothesis

Calcium is the main mineral in bone. The body also draws on the bone store to keep blood calcium steady. These trials did not test how extra calcium changes bone.

A proposed mechanism does not show that the effect happens in people.

See the Safety tab for kidney stones and heart findings from the same trials.

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Preventing fractures

Mixed findings

Across 69 trials, calcium pills alone made little or no difference to any fracture (risk ratio 0.91, just crossing no effect). Calcium taken together with vitamin D lowered the risk of any fracture slightly (0.91). An older review found calcium from food was not linked to fractures.

3 studiesCalcium pills, with and without vitamin D. The 2015 review also looke…

Studies, doses and limits

What the trials found

BMJ 2026 review of 69 trials (153,902 adults): calcium alone, any fracture, 11 trials: RR 0.91 (0.81 to 1.01), moderate certainty; calcium with vitamin D, 15 trials: RR 0.91 (0.84 to 0.99), high certainty; vitamin D alone: RR 1.00. A 2015 BMJ review: calcium pills RR 0.89 for total fracture (0.81 to 0.96) but signs of bias, and no effect in the 4 lowest-bias trials. The WHI trial: total fractures HR 0.96 (0.91 to 1.02), hip HR 0.88 (0.72 to 1.08).

Results

Mixed and small. Calcium alone: little to no effect (moderate certainty). Calcium with vitamin D: a small effect (RR 0.91). Older and less careful analyses showed more benefit; the newer review calls the benefit too small to matter in most people.

Effect sizes as reported

RR 0.91 for any fracture with calcium alone (0.81 to 1.01) and with calcium plus vitamin D (0.84 to 0.99).

Why this rating

Mixed. The reviewers' own GRADE rating is moderate certainty for calcium alone (little to no effect) and high certainty for the combination (RR 0.91). A 2015 review found signs of publication bias.

Form and dose studied

Calcium pills, with and without vitamin D. The 2015 review also looked at calcium from food and milk or dairy and found no link with fractures in most cohort studies.

Who was studied

Mostly community-dwelling adults not at high risk of fractures. Evidence for people at high risk or in care homes was limited. One trial in frail older women in residential care with low calcium and vitamin D intake showed a clear benefit.

Limits

Reviews pool trials of different designs and amounts. The 2026 abstract does not give the amounts used. It does not cover people who eat very little calcium.

Why researchers looked

Weak bones break more easily, so the question people care about is whether extra calcium means fewer broken bones.

Traditional use

Calcium is a nutrient, not a traditional remedy. Long use of calcium-rich foods is not proof that extra calcium pills treat any condition.

Proposed mechanism None

Not tested in these studies.

A proposed mechanism does not show that the effect happens in people.

See the Safety tab: the same trials found more kidney stones.

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Heart disease (a possible harm from pills, not a benefit)

Mixed findings

Calcium from food was not linked to heart disease risk. Some pooled trials of calcium pills found about 15% more heart disease, mostly in healthy postmenopausal women. The question is still debated, and heart events were not the main aim of those trials.

3 studiesCalcium pills (often with vitamin D) in the trials; calcium from food…

Studies, doses and limits

What the trials found

A 2021 meta-analysis of 13 double-blind placebo-controlled trials: cardiovascular disease RR 1.15 (1.06 to 1.25), coronary heart disease RR 1.16 (1.05 to 1.28). A 2020 meta-analysis of 26 cohort studies and 16 trials: dietary calcium no effect (CVD RR 0.96, 0.87 to 1.05); supplements raised heart attack risk (RR 1.14, 1.05 to 1.25). A 10-year US cohort of 5,448 adults: supplement use went with new artery calcification (RR 1.22, 1.07 to 1.39) while higher total intake without pills went with less (top fifth RR 0.73).

Results

Mixed. Pooled trials point to harm from pills in one group, cohort studies of food show no harm, and the answer is still debated.

Effect sizes as reported

Pills: CVD RR 1.15; heart attack RR 1.14. Food: CVD RR 0.96 (not significant). Cohort: new artery calcification with supplement use RR 1.22.

Why this rating

Mixed. Trial meta-analyses of pills agree on higher risk, but the trials were mostly designed for bones, a cohort study is observational, and we did not read the analyses that argue the other way.

Form and dose studied

Calcium pills (often with vitamin D) in the trials; calcium from food in the cohorts.

Who was studied

Healthy postmenopausal women in most trials; adults 45 to 84 in the cohort.

Limits

Heart events were not the planned outcome of most trials. The observational findings cannot show cause. We did not read other analyses of the same trials, so this entry may be one-sided.

Why researchers looked

Calcium helps the heart beat and blood vessels work, and a few trials noticed more heart events in people given pills, so researchers pooled them.

Traditional use

Calcium is a nutrient, not a traditional remedy. Long use of calcium-rich foods is not proof that extra calcium pills treat any condition.

Proposed mechanism Hypothesis

Not established. Some researchers suggest a sudden rise in blood calcium after a pill might affect blood vessels; the abstracts read do not test this.

A proposed mechanism does not show that the effect happens in people.

Talk to your clinician before taking pills if you have heart disease risks. See the Safety tab.

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Kidney stones

Mixed findings

Calcium from food went with fewer kidney stones in a large study of men. Calcium plus vitamin D pills went with more stones in a large trial of women (hazard ratio 1.17). The two results point in opposite directions.

4 studiesCalcium carbonate with vitamin D3 in the WHI trial; calcium from food…

Studies, doses and limits

What the trials found

A cohort of 45,619 men followed 4 years (505 stone cases): highest versus lowest fifth of dietary calcium RR 0.66 (0.49 to 0.90) after adjustment. The WHI trial of 36,282 postmenopausal women: stones 449 on calcium plus vitamin D versus 381 on placebo, HR 1.17 (1.02 to 1.34). A one-year trial in 163 older women found high urine calcium in 30.6% and high blood calcium in 8.8%.

Results

Food: lower stone risk (observational, men). Pills with vitamin D: higher stone risk (randomised trial, women). MedlinePlus says higher amounts over a long time raise stone risk in some people.

Effect sizes as reported

Dietary calcium: RR 0.66 (adjusted). Calcium plus vitamin D pills: HR 1.17.

Why this rating

Mixed. A big randomised trial found more stones with pills, but stones were self-reported and given with vitamin D; the protective finding from food is observational.

Form and dose studied

Calcium carbonate with vitamin D3 in the WHI trial; calcium from food in the cohort.

Who was studied

Postmenopausal women (trial); men aged 40 to 75 (cohort).

Limits

The trial cannot separate calcium from vitamin D. The cohort tested food, not pills, and was in men only.

Why researchers looked

Most kidney stones contain calcium, so people ask whether calcium causes them. Calcium in food can bind oxalate in the gut and lower the amount absorbed.

Traditional use

Calcium is a nutrient, not a traditional remedy. Long use of calcium-rich foods is not proof that extra calcium pills treat any condition.

Proposed mechanism Hypothesis

The 1993 authors note that a high intake of calcium can reduce the urinary excretion of oxalate, which is thought to lower the risk. They did not test it.

A proposed mechanism does not show that the effect happens in people.

See the Safety tab. People who have had a stone should ask their clinician before taking calcium pills.

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Pre-eclampsia in pregnancy

Evidence against

A 2025 Cochrane review of 10 trials in 37,504 pregnant women found calcium pills make little or no difference to pre-eclampsia. The earlier hint of a benefit came from small studies.

1 studiesCalcium supplements in pregnancy; amounts varied by trial and were no…

Studies, doses and limits

What the trials found

Cochrane 2025: against placebo, pre-eclampsia RR 0.83 (0.67 to 1.04; 6 trials, 15,364 women; low certainty). Leaving out small studies, RR 0.92 (0.79 to 1.05; 4 trials, 14,730 women; high certainty). Preterm birth under 37 weeks: RR 0.97 (0.85 to 1.11) in large trials. Adverse effects: RR 2.16 (0.43 to 10.78; very low certainty). Comparing low with high amounts made no difference.

Results

No clear benefit. The overall result leans towards fewer cases but is not significant, and the large trials show little to no difference.

Effect sizes as reported

Pre-eclampsia RR 0.83 (all trials); RR 0.92 (large trials only).

Why this rating

Evidence against a clear benefit. The reviewers rate the large-trial result as high certainty of little to no difference. The Cochrane authors set aside 20 earlier trials, 9 over trustworthiness concerns.

Form and dose studied

Calcium supplements in pregnancy; amounts varied by trial and were not given in the abstract.

Who was studied

Pregnant women, with standard care; baseline calcium intake and pre-eclampsia risk did not change the findings.

Limits

We did not read the WHO guidance on calcium in pregnancy, which may differ for women with very low calcium intake.

Why researchers looked

Earlier small studies suggested that calcium might lower the risk of pre-eclampsia, a serious high-blood-pressure condition of pregnancy.

Traditional use

Calcium is a nutrient, not a traditional remedy. Long use of calcium-rich foods is not proof that extra calcium pills treat any condition.

Proposed mechanism None

Not tested in the abstract.

A proposed mechanism does not show that the effect happens in people.

Pregnant women should not start or stop supplements without asking their midwife or doctor.

Premenstrual symptoms (PMS)

Low certainty

In one 1998 trial of 466 women, premenstrual symptom scores fell 48% with calcium carbonate and 30% with placebo by the third cycle. It is one trial, and there were no repeat trials in the sources we read.

1 studiesCalcium carbonate in the trial.

Studies, doses and limits

What the trials found

A US multicentre double-blind placebo-controlled trial in healthy women aged 18 to 45 with moderate-to-severe premenstrual symptoms: calcium carbonate for 3 cycles. Luteal-phase symptom scores were lower with calcium in cycle 2 (P = .007) and cycle 3 (P below .001); all four symptom groups improved by cycle 3.

Results

A real difference in this trial, but it is one trial from 1998 and symptoms were self-rated.

Effect sizes as reported

Symptom score down 48% with calcium versus 30% with placebo by the third cycle.

Why this rating

Low. A single randomised trial, no repeat in the sources read, self-rated symptoms and funding not stated in the abstract.

Form and dose studied

Calcium carbonate in the trial.

Who was studied

Healthy premenopausal women aged 18 to 45 with moderate-to-severe PMS.

Limits

Other PMS trials of calcium exist (PubMed search showed a 2009 trial and a 2013 pilot) but were not read, so this entry is not a full review.

Why researchers looked

Some researchers have linked premenstrual symptoms to changes in how the body handles calcium.

Traditional use

Calcium is a nutrient, not a traditional remedy. Long use of calcium-rich foods is not proof that extra calcium pills treat any condition.

Proposed mechanism Hypothesis

The authors' idea is that disturbances in calcium regulation may play a part. The trial did not test it.

A proposed mechanism does not show that the effect happens in people.

None specific beyond the Safety tab.

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Connections to the rest of health

How Calcium connects to the rest of our health knowledge graph. Every link says what kind of connection it is. A safety link (for example to the liver) is a warning, not a benefit. A study that merely mentions a topic never creates a benefit link.

Explore every connection: goals, body systems, organs, symptoms, genes

Kind of connection Connected to Search

A. Wellness goals

Connected to	Evidence status	Context	Studies
Bone Health	Moderate certainty	Bone mineral density (bone strength measure): In trials of adults over 50, calcium from food or from pills raised bone density by a small amount, about 0.6 to 1.8%. The gain did not keep growing over time. The reviewers think it is unlikely to cut fractures by a meaningful amount. Preventing fractures: Across 69 trials, calcium pills alone made little or no difference to any fracture (risk ratio 0.91, just crossing no effect). Calcium taken together with vitamin D lowered the risk of any fracture slightly (0.91). An older review found calcium from food was not linked to fractures.	^{11 7 8 2 13 4 5}
Heart Health	Mixed findings	Heart disease (a possible harm from pills, not a benefit): Calcium from food was not linked to heart disease risk. Some pooled trials of calcium pills found about 15% more heart disease, mostly in healthy postmenopausal women. The question is still debated, and heart events were not the main aim of those trials.	^{9 14 1}
Womens Wellness	Evidence against	Pre-eclampsia in pregnancy: A 2025 Cochrane review of 10 trials in 37,504 pregnant women found calcium pills make little or no difference to pre-eclampsia. The earlier hint of a benefit came from small studies.	³
Womens Wellness	Low certainty	Premenstrual symptoms (PMS): In one 1998 trial of 466 women, premenstrual symptom scores fell 48% with calcium carbonate and 30% with placebo by the third cycle. It is one trial, and there were no repeat trials in the sources we read.	¹²

B. Body systems

Connected to	Kind of connection	Evidence status	Context	Studies
Musculoskeletal System	Biological association	Moderate certainty	Studied for: Bone mineral density (bone strength measure) Studied for: Preventing fractures	^{11 7 8 2}
Cardiovascular System	Biological association	Mixed findings	Studied for: Heart disease (a possible harm from pills, not a benefit)	^{9 14 1}
Urinary System	Biological association	Mixed findings	Studied for: Kidney stones	^{13 7 4 5}
Reproductive System	Biological association	Evidence against	Studied for: Pre-eclampsia in pregnancy Studied for: Premenstrual symptoms (PMS)	^{3 12}

E. Health conditions

Connected to	Kind of connection	Evidence status	Context	Studies
Kidney stones	Adverse effect or risk	Documented risk	More reported stones with calcium carbonate plus vitamin D3 in the WHI trial (HR 1.17); calcium from food went with fewer stones in a cohort of men. Preparation or compound: Calcium plus vitamin D pills	^{13 4}
Hypercalcemia (high blood calcium)	Adverse effect or risk	Documented risk	MedlinePlus links milk-alkali syndrome to more than 2,000 mg a day of calcium supplements with high-dose vitamin D. Preparation or compound: Large supplement amounts with high-dose vitamin D	¹⁸
Cardiovascular disease	Adverse effect or risk	Debated	Pooled trials found about 15% more cardiovascular disease with pills in healthy postmenopausal women; food calcium showed no increase. Preparation or compound: Calcium pills	^{9 14}

I. Biomarkers

Connected to	Kind of connection	Evidence status	Context	Studies
Bone mineral density (BMD)	Biological association	Moderate certainty	Measured in trials: raised. A lab change is not the same as a health benefit.	^{11 7}
Any fracture (health outcome)	Biological association	Mixed findings	Measured in trials: mixed. A lab change is not the same as a health benefit.	^{8 7}
Coronary artery calcification (CAC)	Biological association	Low certainty	Measured in trials: raised. A lab change is not the same as a health benefit.	¹
Premenstrual symptom score (17 items)	Biological association	Low certainty	Measured in trials: lowered. A lab change is not the same as a health benefit.	¹²
Urinary tract stone (health outcome)	Biological association	Mixed findings	Measured in trials: mixed. A lab change is not the same as a health benefit.	^{13 4}

K. Biological pathways

Connected to	Kind of connection	Evidence status	Context	Studies
Vitamin D	Preliminary mechanism	Human	MedlinePlus says vitamin D is needed to help the body absorb calcium. Most trials of calcium pills gave it together with vitamin D.	^{17 7}
Iron	Preliminary mechanism	Human	Calcium reduced iron absorption in meal studies.	⁶
Magnesium and phosphorus	Preliminary mechanism	Human	MedlinePlus says milk is also a source of phosphorus and magnesium, which help the body absorb and use calcium.	¹⁷

L. Active compounds

Connected to	Kind of connection	Evidence status	Context	Studies
Calcium carbonate	Biological association	Human	A common supplement form. Studied in the WHI trial, the PMS trial and the levothyroxine study. Absorbed best with food (MedlinePlus).	^{17 7 12 10}
Calcium citrate	Biological association	Human	A common supplement form, absorbed with or without food (MedlinePlus). Used to top up calcium intake in the hypercalciuria trial.	^{16 5}
Calcium in food	Biological association	Human	The main source for most people in the US is dairy. MedlinePlus says spinach, rhubarb and wheat bran bind calcium and limit absorption, and the NHS says the same about spinach.	^{17 19 23}

Each connection is stored with the ingredient, the connected record, the kind of connection, the evidence status, the supporting studies, the preparation or compound involved, and a review date (last reviewed October 26, 2026).

Evidence at a glance: what changed in 5 lab markers

Tap a number to show only those markers. Counts are markers, not people.

Other122

The evidence

The research centre for Calcium. Every record here was checked against PubMed or the publisher's page; we never list a study we have not verified.

Evidence by outcome

Bone mineral density (bone strength measure) 2 studies Moderate certainty Preventing fractures 3 studies Mixed findings Heart disease (a possible harm from pills, not a benefit) 3 studies · 5,448 people in trials Mixed findings Kidney stones 4 studies · 45,619 people in trials Mixed findings Pre-eclampsia in pregnancy 1 study Evidence against Premenstrual symptoms (PMS) 1 study Low certainty

Bars show how many studies we found for each outcome, not how well it works. People are added up across trials only (reviews are not added, to avoid double counting).

How we rate evidence

High certainty

Several well-run trials or reviews agree; further research is unlikely to change the picture much.

Moderate certainty

Good trials point the same way, but some doubt remains.

Low certainty

Small, short or inconsistent trials. The finding could change with better research.

Very low certainty

Very little or very weak human data. Treat as a possibility only.

Mixed findings

Trials or reviews disagree with each other.

Not enough evidence

Not enough human research to say either way.

Evidence against

Good research found no benefit, or found harm.

We take the reviewers' own risk-of-bias and GRADE statements where they exist, and otherwise judge from study size, design, consistency, directness and whether a product matches what is sold. Many publications do not make evidence high. Method note: We started from our existing calcium summary, read NHS and MedlinePlus pages and re-read PubMed abstracts for trials, reviews and cohort studies. Only abstracts were read for studies. Items we could not verify are flagged in the research gaps.

Study library: every study we checked 23

Design Research type Year from Search

Study	Design	People	Product, dose, length	Result
Anderson 2016 Calcium Intake From Diet and Supplements and the Risk of Coronary Artery Calcification and its Progression Among Older Adults: 10-Year Follow-up of the Multi-Ethnic Study of Atherosclerosis (MESA). J Am Heart Assoc · PMID 27729333 · DOI	Cohort study	5448 Adults aged 45 to 84 without known heart disease; 1,567 without baseline artery calcification were followed for new calcification	Calcium from diet (food frequency questionnaire) and supplements (medicine inventory) Calcium as a supplement or from food Total intake by fifths, from about 313 to 2,157 mg a day on average About 10 years · vs Lowest fifth of intake	Highest fifth of total calcium intake: RR 0.73 (0.57 to 0.93) for new calcification. After accounting for total intake, calcium supplement use went with a higher risk: RR 1.22 (1.07 to 1.39). No link between baseline intake and change in calcification in people who already had it. The authors say high total intake went with lower risk, particularly if reached without supplements. Supplement use: RR 1.22 for new calcification (1.07 to 1.39) Methods, limits and funding Outcomes: New coronary artery calcification (a scan marker of plaque) Limitations: Observational, so it cannot show cause. Diet was self-reported once. Calcification is a scan marker, not a heart attack. Supplement users may differ from non-users in other ways. Funding and conflicts: Funding is not stated in the abstract; the PubMed record lists US National Institutes of Health grants. Read: abstract
Bolland 2015 Calcium intake and risk of fracture: systematic review. BMJ · PMID 26420387 · DOI	Systematic review meta analysis	58573 Adults over 50: 2 small trials of dietary calcium, 44 cohort studies, and 26 randomised trials of calcium supplements	Dietary calcium, milk or dairy; or calcium supplements with or without vitamin D Calcium as a supplement or from food Varied by trial Varied · vs Placebo or no supplement	Dietary calcium: most cohort studies found no link with fracture (for example 17 of 21 for hip fracture). Supplement trials: total fracture (20 studies, 58,573 people) RR 0.89 (0.81 to 0.96); vertebral (12 studies) 0.86 (0.74 to 1.00); hip (13 studies) 0.95 (0.76 to 1.18); forearm 0.96 (0.85 to 1.09). Funnel plot and Egger test suggested bias towards supplements in the published data. In the 4 trials at lowest risk of bias (44,505 people) there was no effect at any site. Only one trial, in frail older women in residential care with low calcium and vitamin D, showed a significant reduction. The authors call the evidence that supplements prevent fractures weak and inconsistent. Total fracture RR 0.89 (supplement trials) (0.81 to 0.96) Methods, limits and funding Outcomes: Total, hip, vertebral and forearm fracture Limitations: Older review (searches to September 2014). Signs of publication bias. Cohort studies cannot prove cause. Funding and conflicts: Funded by the Health Research Council of New Zealand. One author has had research grants and fees from several drug companies. Read: abstract
Cluver 2025 Calcium supplementation during pregnancy for preventing hypertensive disorders and related problems. Cochrane Database Syst Rev · PMID 41330480 · DOI	Systematic review meta analysis	37504 Pregnant women in 10 randomised trials (8 against placebo, 2 comparing low and high amounts)	Calcium supplements versus placebo or standard care, and low versus high amounts Calcium as a supplement or from food Varied by trial During pregnancy · vs Placebo or standard care	Versus placebo, pre-eclampsia: RR 0.83 (0.67 to 1.04; 6 trials, 15,364 women; low certainty). Leaving out small studies: RR 0.92 (0.79 to 1.05; 4 trials, 14,730 women; high certainty), little to no difference. Preterm birth before 37 weeks: very uncertain overall, but RR 0.97 (0.85 to 1.11) in the large trials (high certainty). Adverse effects: RR 2.16 (0.43 to 10.78; 2 trials, 714 women; very low certainty). Baseline calcium intake and pre-eclampsia risk did not change the findings. Pre-eclampsia RR 0.83 (all trials) and 0.92 (large trials) (0.67 to 1.04 and 0.79 to 1.05) Methods, limits and funding Outcomes: Pre-eclampsia, maternal death or severe illness, perinatal loss, preterm birth, stillbirth, adverse effects Limitations: The review set aside 20 earlier trials, 9 of them over trustworthiness concerns. Adverse-effect data are very thin. The abstract does not say which low-calcium populations were included. Funding and conflicts: Part-funded by the World Health Organization. Authors report no relevant interests. Registration: PROSPERO CRD42024623889 Read: abstract
Curhan 1993 A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones. N Engl J Med · PMID 8441427 · DOI	Cohort study	45619 Men aged 40 to 75 with no history of kidney stones	Dietary calcium from food (food frequency questionnaire) Calcium as a supplement or from food Fifths of dietary calcium intake 4 years of follow-up (505 stone cases) · vs Lowest fifth of intake	Highest versus lowest fifth of dietary calcium: RR 0.56 (0.43 to 0.73), and 0.66 (0.49 to 0.90) after adjusting for other risks. Higher animal protein went with more stones (RR 1.33, 1.00 to 1.77); more potassium (RR 0.49) and more fluid (RR 0.71) went with fewer. The authors conclude a high dietary calcium intake decreases stone risk. Highest vs lowest fifth of dietary calcium: RR 0.66 (adjusted) (0.49 to 0.90) Methods, limits and funding Outcomes: Symptomatic kidney stones Limitations: Men only, from 1993, food intake self-reported, observational. It tested calcium from food, not calcium pills. Funding and conflicts: Funding is not stated in the abstract; the PubMed record lists US National Institutes of Health grants. Read: abstract
Gallagher 2014 Incidence of hypercalciuria and hypercalcemia during vitamin D and calcium supplementation in older women. Menopause · PMID 24937025 · DOI	Randomized trial	163 163 white women aged 57 to 90 with low vitamin D levels	Vitamin D at different amounts plus calcium citrate tablets to reach a total calcium intake of about 1,200 mg a day Calcium as a supplement or from food Vitamin D 400 to 4,800 IU a day; total calcium about 1,200 mg a day 1 year · vs Placebo for vitamin D (all groups had the calcium)	High blood calcium (above 10.2 mg/dL) occurred in 8.8% of women and high urine calcium (above 300 mg a day) in 30.6%. Hypercalciuria was transient in half and recurrent in the other half. No link with vitamin D dose was found, and hypercalciuria was equally common in the placebo group. The authors say it is unclear whether calcium, vitamin D or both cause it, and note the possible relevance to the kidney stones seen in the WHI trial. High blood calcium 8.8%; high urine calcium 30.6% (Not given in the abstract) Methods, limits and funding Outcomes: Blood calcium and 24-hour urine calcium every 3 months Limitations: White women only, one year. Everyone had the calcium, so calcium effects cannot be separated from vitamin D. Funding and conflicts: Funding is not stated in the abstract; the PubMed record lists a US National Institute on Aging grant. Read: abstract
Hallberg 1991 Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. Am J Clin Nutr · PMID 1984335 · DOI	Controlled experiment	126 126 human volunteers	Calcium added to wheat rolls or given as milk, cheese or calcium chloride, with iron Calcium as a supplement or from food 40 to 600 mg calcium per test meal Single test meals · vs Meals without added calcium	Calcium cut iron absorption in a dose-related way up to 300 mg. Iron absorption fell 50 to 60% at 300 to 600 mg calcium. 165 mg calcium as milk, cheese or calcium chloride also cut absorption by 50 to 60%, and it lowered heme-iron absorption as well, suggesting an effect in the gut lining. The authors note these calcium amounts are common in normal meals. Iron absorption down 50 to 60% with 165 to 600 mg calcium in a meal (Not given in the abstract) Methods, limits and funding Outcomes: Iron absorption (non-heme and heme) Limitations: Single meals from 1991, not long-term iron status. The abstract does not say whether the effect lasts over weeks of eating. Funding and conflicts: Funding is not stated in the abstract. Read: abstract
Jackson 2006 Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med · PMID 16481635 · DOI	Randomized trial	36282 Healthy postmenopausal women aged 50 to 79 in the Women's Health Initiative	Calcium carbonate with vitamin D3 versus placebo Calcium as a supplement or from food 1,000 mg elemental calcium as calcium carbonate with 400 IU vitamin D3 daily Average 7.0 years · vs Placebo	Hip bone density 1.06% higher with calcium plus vitamin D (P below 0.01). Hip fracture hazard ratio 0.88 (0.72 to 1.08); clinical spine fracture 0.90 (0.74 to 1.10); total fractures 0.96 (0.91 to 1.02). Kidney stones (renal calculi) were more common: hazard ratio 1.17 (1.02 to 1.34). Counting only women who kept taking the study pills, the hip fracture hazard ratio was 0.71 (0.52 to 0.97). Hip fracture HR 0.88 (not significant); kidney stones HR 1.17 (Hip fracture 0.72 to 1.08; kidney stones 1.02 to 1.34) Methods, limits and funding Outcomes: Hip fracture, spine fracture, total fractures, hip bone density, kidney stones Limitations: Many women were already taking calcium and vitamin D outside the trial, which can blur the difference. The adherence analysis is not the main result. Calcium was given together with vitamin D, so the effect of calcium alone cannot be separated. Funding and conflicts: Funding is not stated in the abstract; the PubMed record is tagged as supported by the US National Institutes of Health. Registration: NCT00000611 Read: abstract
Massé 2026 Calcium, vitamin D, or combined supplementation to prevent fractures and falls: systematic review and meta-analysis. BMJ · PMID 42161415 · DOI	Systematic review meta analysis	153902 Adults 18 and over not taking osteoporosis drugs; 87% community dwelling, 73% not at high risk of fractures or falls	Calcium alone, vitamin D alone, or both, versus placebo or no treatment Calcium as a supplement or from food Varied by trial (69 trials) · vs Placebo or no treatment	69 trials, 153,902 people. Any fracture: calcium alone (11 trials, 9,067 people) risk ratio 0.91 (0.81 to 1.01), moderate certainty; vitamin D alone (36 trials, 92,045 people) 1.00 (0.95 to 1.06), high certainty; both together (15 trials, 51,126 people) 0.91 (0.84 to 0.99), high certainty. Little to no effect on other fracture and fall outcomes. Evidence for high-risk people and care-home residents was limited for calcium alone and for the combination. Calcium alone, any fracture: RR 0.91 (0.81 to 1.01) Methods, limits and funding Outcomes: Any fracture (main); hip, non-vertebral and vertebral fracture; falls Limitations: Most trials were in community-dwelling people who were not at high risk. Evidence was limited for people at high risk and in residential care. The abstract does not give the amounts used. Funding and conflicts: No industry funding declared for the review. Some authors report payments from non-industry organisations for osteoporosis education. Registration: PROSPERO CRD42023483915 Read: abstract
Myung 2021 Calcium Supplements and Risk of Cardiovascular Disease: A Meta-Analysis of Clinical Trials. Nutrients · PMID 33530332 · DOI	Systematic review meta analysis	43178 13 double-blind placebo-controlled trials; the increase was seen in healthy postmenopausal women	Calcium supplements versus placebo Calcium as a supplement or from food Subgroups: 1,000 mg a day of supplementary calcium; or 700 to 1,000 mg a day total Varied by trial · vs Placebo	Calcium supplements raised the risk of cardiovascular disease (RR 1.15, 1.06 to 1.25) and coronary heart disease (RR 1.16, 1.05 to 1.28) in double-blind placebo-controlled trials, specifically in healthy postmenopausal women. The authors summarise it as about 15% higher cardiovascular risk in that group. CVD RR 1.15; CHD RR 1.16 (1.06 to 1.25; 1.05 to 1.28) Methods, limits and funding Outcomes: Cardiovascular disease, coronary heart disease, cerebrovascular disease Limitations: Pooled trials that were mostly designed for bone outcomes, not heart outcomes. Heart events were not the main aim of the trials. The abstract says earlier reviews disagreed. Funding and conflicts: The authors declare no conflict of interest. Read: abstract
Singh 2000 Effect of calcium carbonate on the absorption of levothyroxine. JAMA · PMID 10838651 · DOI	Cohort study	20 20 people with hypothyroidism on stable long-term levothyroxine (11 men), at one Veterans Affairs medical centre	Calcium carbonate taken with levothyroxine Calcium as a supplement or from food 1,200 mg elemental calcium a day as calcium carbonate, taken with the levothyroxine 3 months, then 2 months after stopping calcium · vs Each person's own levels on levothyroxine alone	Mean free T4 fell from 17 to 15 pmol/L on calcium and returned to 18 pmol/L after stopping (P below .001). Mean TSH rose from 1.6 to 2.7 mIU/L on calcium and fell to 1.4 mIU/L after stopping (P = .008). 20% of people had TSH above the normal range on calcium (highest 7.8). A lab test showed levothyroxine sticks to calcium carbonate in acid. Free T4 down and TSH up while calcium and levothyroxine were taken together (Not given in the abstract) Methods, limits and funding Outcomes: Free and total thyroxine (T4), T3 and thyrotropin (TSH) Limitations: Only 20 people, no placebo group, calcium and levothyroxine taken together. It does not test spacing the two apart. Funding and conflicts: Funding is not stated in the abstract. Read: abstract
Tai 2015 Calcium intake and bone mineral density: systematic review and meta-analysis. BMJ · PMID 26420598 · DOI	Systematic review meta analysis	Adults over 50 in randomised trials: 15 trials of dietary sources of calcium (1,533 people) and 51 of calcium supplements (12,257 people)	Calcium from food sources or calcium supplements, with or without vitamin D Calcium as a supplement or from food Varied by trial Up to over 2.5 years · vs Control groups without extra calcium	Dietary sources raised bone density by 0.6 to 1.0% at the total hip and whole body at one year and 0.7 to 1.8% at two years; no effect at the forearm. Supplements raised bone density by 0.7 to 1.8% at all five sites; the gain at later times was about the same as at one year. The authors conclude the increases are small, do not keep growing and are unlikely to lead to a clinically significant fall in fracture risk. Bone density about 0.7 to 1.8% higher with supplements (Not given in the abstract) Methods, limits and funding Outcomes: Bone mineral density at the spine, hip, femoral neck, whole body and forearm Limitations: Bone density is a stand-in measure, not fractures. The abstract reports 59 eligible trials but also 15 and 51 trials by type, so some trials counted in both groups. Funding and conflicts: Study funding is not stated in the record. One author holds a New Zealand research fellowship; another has had research grants and fees from several drug companies (Merck, Amgen, Lilly, Novartis). Read: abstract
Thys-Jacobs 1998 Calcium carbonate and the premenstrual syndrome: effects on premenstrual and menstrual symptoms. Am J Obstet Gynecol · PMID 9731851 · DOI	Randomized trial	497 Healthy premenopausal women aged 18 to 45 with moderate-to-severe premenstrual symptoms; 466 valid for analysis	Calcium carbonate versus placebo Calcium as a supplement or from food 1,200 mg elemental calcium a day as calcium carbonate 3 menstrual cycles · vs Placebo	Symptom scores were lower with calcium in the second (P = .007) and third (P below .001) treatment cycles. By the third cycle, total symptom scores fell 48% from baseline with calcium and 30% with placebo. All four symptom groups (negative mood, water retention, food cravings, pain) were reduced. Symptom score down 48% with calcium versus 30% with placebo (Not given in the abstract) Methods, limits and funding Outcomes: Total symptom score (17 symptoms) in the luteal phase Limitations: One trial from 1998, three cycles, symptoms self-rated. Funding is not stated in the abstract. The abstract does not give group sizes or side effects. Funding and conflicts: Funding is not stated in the abstract. Read: abstract
Wallace 2011 Urinary tract stone occurrence in the Women's Health Initiative (WHI) randomized clinical trial of calcium and vitamin D supplements. Am J Clin Nutr · PMID 21525191 · DOI	Randomized trial	36282 Postmenopausal women aged 50 to 79 at 40 WHI centres (18,176 supplement, 18,106 placebo)	Calcium carbonate with vitamin D3 versus placebo Calcium as a supplement or from food 500 mg calcium carbonate with 200 IU vitamin D3 twice daily (1,000 mg and 400 IU a day) Average 7.0 years · vs Matching placebo	Stones were reported by 449 women on calcium plus vitamin D and 381 on placebo: hazard ratio 1.17 (1.02 to 1.34). The excess did not differ by age, body size, diet or other risk factors. Baseline total calcium intake and baseline supplement use were not linked to stone risk. Results were similar when women with poor adherence were left out. Stones HR 1.17 (1.02 to 1.34) Methods, limits and funding Outcomes: Self-reported, clinically diagnosed urinary tract stones Limitations: Stones were self-reported. Calcium was given with vitamin D, so calcium alone cannot be separated. Postmenopausal women only. Funding and conflicts: Funding is not stated in the abstract; the PubMed record lists US National Institutes of Health contracts for the Women's Health Initiative. Registration: NCT00000611 Read: abstract
Yang 2020 The Evidence and Controversy Between Dietary Calcium Intake and Calcium Supplementation and the Risk of Cardiovascular Disease: A Systematic Review and Meta-Analysis of Cohort Studies and Randomized Controlled Trials. J Am Coll Nutr · PMID 31625814 · DOI	Systematic review meta analysis	26 prospective cohort studies (diet) and 16 randomised trials (supplements)	Dietary calcium (cohorts) and calcium supplements with or without vitamin D (trials) Calcium as a supplement or from food Dietary intakes of 200 to 1,500 mg a day in the cohorts Varied · vs Lowest intake or placebo	Dietary calcium did not change risk: RR for CVD 0.96 (0.87 to 1.05), CHD 0.98 (0.88 to 1.08), stroke 0.94 (0.85 to 1.04). In trials, supplements raised coronary heart disease risk (RR 1.08, reported with CI 1.02 to 1.22) and, for calcium alone, 1.20 (1.08 to 1.33). Heart attack risk was 14% higher (RR 1.14, 1.05 to 1.25) and 21% higher for calcium alone (RR 1.21, 1.08 to 1.35). Supplements: heart attack RR 1.14. Dietary calcium: no effect (Heart attack 1.05 to 1.25) Methods, limits and funding Outcomes: Cardiovascular disease, coronary heart disease, stroke, heart attack Limitations: Searches to March 2019. The trials were mainly designed for bone outcomes. The abstract's CHD figures (RR 1.08 with CI 1.02 to 1.22) are quoted as printed. Funding and conflicts: Not stated in the abstract. Read: abstract
s-efsa-calcium-ul EFSA: Scientific opinion on the tolerable upper intake level of calcium (2012) · Source	Regulatory opinion n/a			Methods, limits and funding Read: not-read
s-medlineplus-calcium MedlinePlus: Calcium (health topic) · Source	Health agency n/a			Methods, limits and funding Read: full-text
s-medlineplus-calcium-diet MedlinePlus Medical Encyclopedia: Calcium in diet · Source	Health agency n/a			Methods, limits and funding Read: full-text
s-medlineplus-hypercalcemia MedlinePlus Medical Encyclopedia: Hypercalcemia · Source	Health agency n/a			Methods, limits and funding Read: full-text
s-nhs-calcium NHS: Calcium (vitamins and minerals) · Source	Health agency n/a			Methods, limits and funding Read: full-text
s-nhs-ciprofloxacin NHS: Ciprofloxacin · Source	Health agency n/a			Methods, limits and funding Read: full-text
s-nhs-levothyroxine NHS: Levothyroxine · Source	Health agency n/a			Methods, limits and funding Read: full-text
s-nih-ods-calcium NIH Office of Dietary Supplements: Calcium fact sheets (health professional and consumer) · Source	Health agency n/a			Methods, limits and funding Read: not-read
s-tofu-usda USDA values summarised in our tofu page (regular tofu set with calcium sulfate) · Source	Database summary n/a			Methods, limits and funding Read: summary-only

What we do not know yet

We did not read analyses that find no heart harm from calcium pills, so the heart findings may be one-sided.
No trial in children, teenagers or men was read; most trials were in older and postmenopausal women.
Calcium plus vitamin D was given together in most trials, so the effect of calcium alone is often unclear.
Only one premenstrual syndrome trial was read; colon polyp, blood pressure and dental findings were not covered.
Breastfeeding, liver safety, contamination and product quality were not researched.
The NIH Office of Dietary Supplements and EFSA pages were blocked, so their food values and upper limits were not read; the tofu value is carried over from our tofu page.
The abstracts read do not give the calcium amounts used in several trials.

How new research is added

New studies are found by a saved PubMed search and placed in a review queue before the page changes.

Lab markers

What changed in blood or saliva tests during trials. A changed number is not the same as feeling or being healthier, and we give no "normal ranges".

Other 5

▲

Bone mineral density (BMD)

Raised in trials

A scan measure of how much mineral is in a bone. Low density goes with weaker bones.

Moderate certainty

Results and what it means

Why it was measured

It is the usual way to track bone strength in trials.

Statistical vs. clinical meaning

A small rise that does not keep growing; the reviewers think it is unlikely to cut fractures meaningfully.

Safety implication

None specific for this marker.

Study	Result	People	Product
¹¹	Calcium pills raised BMD by 0.7 to 1.8% at all five sites; calcium from food by 0.6 to 1.8% at hip, whole body (and spine and femoral neck at 2 years)	Adults over 50 in 59 randomised trials	Calcium from food or pills, with or without vitamin D
⁷	Hip BMD 1.06% higher than placebo (P below 0.01)	36,282 postmenopausal women, about 7 years	Calcium carbonate with vitamin D3 daily

^{11 7}

↕

Any fracture (health outcome)

Mixed results in trials

A broken bone of any kind during the trial.

Mixed findings

Results and what it means

Why it was measured

It is the outcome that matters to patients.

Statistical vs. clinical meaning

Little to no effect for calcium alone; a small effect for the combination.

Safety implication

See the Safety tab for kidney stones.

Study	Result	People	Product
⁸	Calcium alone RR 0.91 (0.81 to 1.01), moderate certainty; calcium with vitamin D RR 0.91 (0.84 to 0.99), high certainty	69 trials, 153,902 adults, mostly not at high risk	Calcium, vitamin D or both versus placebo or no treatment
⁷	Total fractures HR 0.96 (0.91 to 1.02); hip fracture HR 0.88 (0.72 to 1.08)	36,282 postmenopausal women	Calcium carbonate with vitamin D3 versus placebo, about 7 years

^{8 7}

▲

Coronary artery calcification (CAC)

Raised in trials

Calcium deposits in the heart's arteries, measured on a CT scan. More deposits go with more plaque.

Low certainty

Results and what it means

Why it was measured

It is a scan marker used to track hardening of the arteries.

Statistical vs. clinical meaning

Observational link between pills and a scan marker. It does not show heart attacks or cause.

Safety implication

See Safety tab.

Study	Result	People	Product
¹	Supplement use went with higher risk of new calcification (RR 1.22, 1.07 to 1.39); the highest fifth of total intake went with lower risk (RR 0.73, 0.57 to 0.93)	1,567 adults without baseline calcification, 10-year follow-up	Calcium from diet (questionnaire) and supplements (medicine list); observational

▼

Premenstrual symptom score (17 items)

Lowered in trials

A daily self-rating of 17 symptoms such as mood, water retention, cravings and pain.

Low certainty

Results and what it means

Why it was measured

It is how PMS trials measure change.

Statistical vs. clinical meaning

One trial; symptoms are self-rated.

Safety implication

None specific.

Study	Result	People	Product
¹²	Total symptom score down 48% from baseline with calcium versus 30% with placebo by cycle 3	466 women with moderate-to-severe PMS	Calcium carbonate for 3 cycles

¹²

↕

Urinary tract stone (health outcome)

Mixed results in trials

A stone in the kidney or urinary tract that a patient reported was diagnosed by a clinician.

Mixed findings

Results and what it means

Why it was measured

It is a main harm outcome tracked for calcium pills.

Statistical vs. clinical meaning

Pills with vitamin D raised stone reports in a trial, while food calcium went with fewer stones in a cohort.

Safety implication

Ask a clinician before using calcium pills if you have had stones.

Study	Result	People	Product
¹³	449 stones with calcium plus vitamin D versus 381 with placebo; HR 1.17 (1.02 to 1.34)	36,282 postmenopausal women, about 7 years	Calcium carbonate with vitamin D3 daily
⁴	Highest versus lowest fifth of dietary calcium: RR 0.66 (0.49 to 0.90) adjusted	45,619 men, 4 years	Dietary calcium from food; observational

^{13 4}

Safety

What studies and official sources say about side effects, medicines and who should check first. Each point shows what kind of evidence it comes from.

Main points to know 5

Kidney stones: calcium plus vitamin D pills raised stone reports in a large trial. In the Women's Health Initiative trial of 36,282 postmenopausal women, calcium carbonate with vitamin D3 for about 7 years went with more reported kidney stones than placebo (449 versus 381; hazard ratio 1.17, 1.02 to 1.34). MedlinePlus also says higher amounts over a long time raise the risk of stones in some people. In a separate study of men, calcium from food went with fewer stones. If you have had a stone, ask your clinician before using calcium pills. Randomized trial ^{13 7 17 4}
Heart disease: pooled trials of calcium pills found more heart events. A 2021 meta-analysis of 13 double-blind placebo-controlled trials found calcium supplements raised cardiovascular disease risk by about 15% (RR 1.15, 1.06 to 1.25), specifically in healthy postmenopausal women. A 2020 meta-analysis found no effect from calcium eaten in food but a higher heart attack risk with pills (RR 1.14). This is debated and heart events were not the main aim of the trials. Ask your clinician if you have heart disease or risk factors. Meta-analysis ^{9 14 1}
Too much calcium: high blood calcium and milk-alkali syndrome. Very high calcium in the blood (hypercalcemia) can cause nausea, thirst, constipation, muscle weakness and confusion, and severe cases need hospital care. MedlinePlus says it is most often caused by an overactive parathyroid gland, and that taking more than 2,000 mg a day of calcium supplements (its page says "calcium bicarbonate") together with high doses of vitamin D can cause it too (milk-alkali syndrome). In a one-year trial in 163 older women, calcium and vitamin D supplements were followed by high urine calcium in 30.6% and high blood calcium in 8.8%, and the authors could not say whether calcium, vitamin D or both caused it. Authoritative guidance ^{18 5}
Everyday side effects: gas, bloating, constipation. MedlinePlus says calcium supplements might cause gas, bloating and constipation in some people. The NHS says more than 1,500 mg a day could lead to stomach pain and diarrhoea. Authoritative guidance ^{16 19}
Medicines that do not mix well with calcium. Calcium carbonate lowered levothyroxine absorption in a 20-person study, and the NHS lists calcium supplements among things that may not mix well with it. The NHS says to leave at least 2 hours between ciprofloxacin and dairy foods or drinks with added calcium. Calcium also cut iron absorption in a meal study. Ask a pharmacist about spacing. Controlled human study ^{10 21 20 6}

Side effects 4

Effect	How often	Severity and recovery	Evidence
Kidney stones (urinary tract stones) Stones were self-reported. Calcium was given with vitamin D, so calcium alone cannot be separated.	WHI trial, about 7 years: 449 stones among 18,176 women on calcium plus vitamin D versus 381 among 18,106 on placebo (HR 1.17, 1.02 to 1.34)	painful; can need treatment Not described in the abstract.	Randomized trial ^{13 7}
High urine calcium and high blood calcium White women only; calcium and vitamin D effects not separated.	One-year trial in 163 older women with total calcium about 1,200 mg a day plus vitamin D: high urine calcium 30.6%, high blood calcium 8.8%; also in the placebo group for urine calcium	usually found on tests; high blood calcium can cause symptoms Hypercalciuria was transient in half the women and recurrent in the other half.	Randomized trial ⁵
Gas, bloating, constipation; stomach pain and diarrhoea at high amounts Spreading doses out, taking with meals or switching form is suggested by MedlinePlus.	Not stated by the sources read	mild Not described.	Authoritative guidance ^{16 19}
More heart events with calcium pills in pooled trials Debated. Cohort studies of calcium from food showed no increase in risk.	Cardiovascular disease RR 1.15 (1.06 to 1.25), coronary heart disease RR 1.16 (1.05 to 1.28) in 13 trials	can be serious Not applicable	Meta-analysis ^{9 14}

Liver

No liver injury from calcium was found in the sources read. A liver-safety source such as LiverTox was not searched, so this is a gap, not proof of safety.

How often. No frequency was found.

Monitoring. No agency monitoring advice was found.

This is a safety connection. It is not evidence that Calcium supports liver health.

Medicines and supplements 5

With	Why it matters	Importance	Evidence	What to do
Levothyroxine (thyroid hormone) prescription	Levothyroxine sticks to calcium carbonate in acid in a lab test, which may reduce how much is absorbed.	In 20 people with hypothyroidism taking calcium carbonate with levothyroxine, mean free T4 fell (17 to 15 pmol/L) and mean TSH rose (1.6 to 2.7 mIU/L); 20% had TSH above the normal range. Both returned towards baseline after stopping calcium. The NHS lists calcium and iron supplements among things that may not mix well with levothyroxine.	Controlled human study Moderate certainty ^{10 21}	Ask your pharmacist or prescriber how far apart to take them. Do not change your thyroid medicine without advice.
Ciprofloxacin (antibiotic) prescription	The NHS says the calcium in dairy foods and calcium-added drinks can stop the body taking in all of the medicine.	The NHS says to leave a gap of at least 2 hours between ciprofloxacin and dairy foods or drinks with added calcium. The page is about food and drinks; it does not name calcium pills, so the same caution for pills is an inference from the mechanism.	Authoritative guidance Moderate certainty ²⁰	Ask a pharmacist before taking calcium pills with this antibiotic.
Iron (food and supplements) supplement	Calcium lowered both non-heme and heme iron absorption in meal studies, which suggests an effect in the gut lining.	In 126 volunteers, calcium cut iron absorption by 50 to 60% at 165 to 600 mg per test meal. These were single meals; the abstract does not show effects on iron stores over weeks.	Controlled human study Moderate certainty ⁶	If you are iron-deficient or take iron pills, ask a clinician or pharmacist about timing.
Thiazide diuretics (water pills) and lithium prescription	MedlinePlus lists these medicines among causes of high blood calcium.	The source lists them as causes of raised blood calcium; it does not describe a calcium-pill interaction directly.	Authoritative guidance Not enough evidence ¹⁸	If you take either medicine, ask your clinician before adding calcium pills.
High-dose vitamin D supplement	Too much vitamin D in the blood raises blood calcium; MedlinePlus links milk-alkali syndrome to large calcium supplement amounts together with high-dose vitamin D.	MedlinePlus says milk-alkali syndrome most often occurs with more than 2,000 mg a day of calcium supplements taken with high doses of vitamin D.	Authoritative guidance Moderate certainty ¹⁸	Ask a clinician before taking large amounts of calcium and vitamin D together.

Who should check first 5

Situation	Advice	Evidence
High blood calcium (hypercalcemia), an overactive parathyroid gland, or a clinician's advice to limit calcium	Do not add calcium pills without medical advice. MedlinePlus says people with kidney stones should have tests for hypercalcemia.	Authoritative guidance ¹⁸
History of kidney stones	Ask your clinician before using calcium pills. A large trial found more stones with calcium plus vitamin D, and MedlinePlus says higher amounts over a long time raise stone risk in some people.	Randomized trial ^{13 17}
Kidney disease	MedlinePlus lists chronic kidney disease or kidney failure among causes of high blood calcium. Ask your kidney doctor before using calcium products.	Authoritative guidance ¹⁸
Heart disease or high heart risk	Pooled trials found more heart events with calcium pills. Ask your clinician before starting pills.	Meta-analysis ^{9 14}
Taking levothyroxine, ciprofloxacin, iron, thiazide diuretics or lithium	See the interactions list; ask a pharmacist about timing.	Authoritative guidance ^{21 20 18}

Overdose, product quality and regulators

Summary. No toxicity from calcium in ordinary food was found in the sources read. Excess comes mainly from large supplement amounts, especially with vitamin D, and from medical causes of high blood calcium.

Animal. Not reviewed.

Human doses studied. In trials, adults were given amounts as listed in the studies table; the official upper limits are in the regulatory list.

High dose or long term signals. Kidney stones with calcium plus vitamin D in a large trial; more heart events in pooled trials of pills; hypercalcemia and milk-alkali syndrome at very high supplement amounts with vitamin D.

Overdose response. Severe hypercalcemia can need hospital treatment with fluids through a vein (MedlinePlus). For a large accidental swallow of a calcium or vitamin D product, call Poison Help (1-800-222-1222 in the US) or your local emergency number.

Genotoxicity. Not researched.

Product quality (label accuracy, contaminants such as lead, whether a tablet dissolves) was not researched on this page. A PubMed search for lead in calcium supplements returned no relevant study to read.

Body	Action	Date
NHS (UK) (United Kingdom)	Calcium page: more than 1,500 mg a day could lead to stomach pain and diarrhoea; 1,500 mg or less a day is unlikely to cause harm; a lack of calcium could lead to rickets in children and osteomalacia or osteoporosis later. source	2020-08-03
MedlinePlus (US National Library of Medicine), citing the US Dietary Reference Intakes (United States)	Calcium in diet: up to 2,500 mg a day from food and supplements appears to be safe for adults 19 to 50; 2,000 mg a day for adults over 50; 2,500 to 3,000 mg a day for children and adolescents. Higher amounts over a long time raise the risk of kidney stones in some people. source	2025-01-21
MedlinePlus (US National Library of Medicine) (United States)	Hypercalcemia page: causes include overactive parathyroid glands, too much vitamin D, kidney disease, thiazide diuretics, lithium and milk-alkali syndrome; milk-alkali syndrome most often occurs with more than 2,000 mg a day of calcium supplements with high-dose vitamin D. source	2026-01-25
NHS (UK) (United Kingdom)	Ciprofloxacin: leave at least 2 hours between the antibiotic and dairy products or drinks with added calcium. source	2026-10-09
NHS (UK) (United Kingdom)	Levothyroxine: calcium and iron supplements and antacids may not mix well with it. source	2026-07-06

When to get medical help

Confusion, severe muscle weakness, repeated vomiting or extreme thirst after high amounts of calcium or vitamin D (possible high blood calcium): seek urgent medical care or call 911 or your local emergency number
Severe pain in the side or back, or blood in the urine (possible kidney stone): contact a doctor urgently
A child or adult has swallowed a large amount of a calcium or vitamin D product by accident: call Poison Help at 1-800-222-1222 (US) or your local emergency number

Stop using the product and contact a clinician or emergency service. In the US, Poison Control is 1-800-222-1222.

General information only, not medical advice. Ask a clinician or pharmacist before use, especially with medicines, in pregnancy or with a health condition. See the Natural Remedy Safety guide.

Frequently asked questions

What does calcium do in the body?

MedlinePlus says almost all of your calcium is stored in bones and teeth, where it keeps them strong, and that it helps muscles move, nerves carry messages, blood vessels move blood and hormones be released. The NHS adds a normal heartbeat and blood clotting.

^{16 19}

Which foods are high in calcium?

Dairy foods are the main source for most people in the US. MedlinePlus gives about 300 mg in 8 oz of nonfat milk or fortified soy milk, 1.5 oz of cheese or 6 oz of yogurt, and about 325 mg in 3 oz of sardines with bones. Other sources include kale, broccoli, bok choy, almonds, tahini and calcium-enriched foods. Spinach and rhubarb bind calcium, so they give less.

^{16 17 19}

Do calcium supplements prevent fractures?

Mostly no. A 2026 BMJ review of 69 trials found calcium alone made little to no difference to any fracture (RR 0.91, 0.81 to 1.01, moderate certainty). Calcium with vitamin D gave a small reduction (RR 0.91, 0.84 to 0.99). Evidence for people at high risk or in care homes was limited.

^{8 2}

Does calcium make bones denser?

A little. In adults over 50, calcium from food or pills raised bone density by about 0.6 to 1.8%, but the gain did not keep growing and the reviewers think it is unlikely to reduce fractures meaningfully.

^{11 7}

Do calcium pills cause kidney stones?

They may. In the Women's Health Initiative trial of 36,282 postmenopausal women, calcium carbonate with vitamin D3 went with more reported stones (HR 1.17, 1.02 to 1.34). Calcium from food went with fewer stones in a large study of men. If you have had a stone, ask your clinician before using pills.

^{13 4 17}

Are calcium supplements bad for the heart?

Possibly, but it is debated. A 2021 meta-analysis of 13 trials found about 15% more cardiovascular events with pills in healthy postmenopausal women. A 2020 analysis found calcium from food did not change risk. Heart events were not the main aim of the trials. Ask your clinician if you have heart disease.

^{9 14 1}

Does calcium prevent pre-eclampsia?

Not clearly. A 2025 Cochrane review of 10 trials found little to no difference (RR 0.83, 0.67 to 1.04, low certainty), and high-certainty evidence of little to no difference when small studies were left out. Ask your midwife or doctor about your own care.

Does calcium help with PMS?

One 1998 trial of 466 women found symptom scores fell 48% with calcium carbonate and 30% with placebo by the third cycle. It is one trial with self-rated symptoms, so the evidence is low certainty.

¹²

Does calcium interact with medicines?

Yes, with some. Calcium carbonate lowered levothyroxine absorption in a 20-person study and the NHS lists calcium among things that may not mix well with it. The NHS says to leave at least 2 hours between ciprofloxacin and dairy or calcium-added drinks. Thiazide diuretics and lithium can raise blood calcium. Ask a pharmacist about timing.

^{10 21 20 18}

Does calcium affect iron absorption?

In meal studies in 126 volunteers, calcium cut iron absorption by 50 to 60% at 165 to 600 mg per meal, including iron from meat. These were single meals. If you have low iron, ask a clinician or pharmacist.

⁶

Can you get too much calcium?

Yes. Very high blood calcium can cause nausea, thirst, constipation, muscle weakness and confusion. MedlinePlus says it is most often due to the parathyroid glands but heavy supplement use with high-dose vitamin D can cause it. MedlinePlus (citing US reference intakes) says up to 2,500 mg a day from food and supplements appears safe for adults 19 to 50, and the NHS says 1,500 mg or less is unlikely to cause harm. These are regulators' statements, not advice to take that amount.

^{18 17 19}

Is calcium from food different from calcium pills?

In the studies read, yes. Food calcium showed no rise in heart risk and went with fewer kidney stones; pills went with more stones in a trial and more heart events in pooled trials. Food also brings protein and other nutrients. Cohort studies cannot prove cause.

^{14 4 13}

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Editorial transparency

Written by: AlternativeMed editorial team. Research compiled with AI assistance, then checked against the sources listed.
Medical review: None. This page has not been reviewed by a physician or pharmacist.
First published: 2026-10-26
Last substantive update: 2026-10-26
How we research: We started from our existing calcium summary, read NHS and MedlinePlus pages and re-read PubMed abstracts for trials, reviews and cohort studies. Only abstracts were read for studies. Items we could not verify are flagged in the research gaps.
How we rate evidence: See "How we rate evidence" in the Evidence tab.
Independence: No product is promoted on this page. No affiliate links are shown. Some authors of studies cited report grants or fees from drug companies or non-industry groups; this is stated where each study is used. The 2026 BMJ review declares no industry funding.

Sources 23

Each source was opened and checked. Where we read only the abstract, it says so. The small numbers in the text point here.

Anderson JJ, Kruszka B, Delaney JA, He K, Burke GL, Alonso A. Calcium Intake From Diet and Supplements and the Risk of Coronary Artery Calcification and its Progression Among Older Adults: 10-Year Follow-up of the Multi-Ethnic Study of Atherosclerosis (MESA).. J Am Heart Assoc. 2016. PMID 27729333. DOI 10.1161/JAHA.116.003815. (abstract)
Bolland MJ, Leung W, Tai V, Bastin S, Gamble GD, Grey A. Calcium intake and risk of fracture: systematic review.. BMJ. 2015. PMID 26420387. DOI 10.1136/bmj.h4580. (abstract)
Cluver CA, Rohwer C, Rohwer AC. Calcium supplementation during pregnancy for preventing hypertensive disorders and related problems.. Cochrane Database Syst Rev. 2025. PMID 41330480. DOI 10.1002/14651858.CD001059.pub6. (abstract)
Curhan GC, Willett WC, Rimm EB, Stampfer MJ. A prospective study of dietary calcium and other nutrients and the risk of symptomatic kidney stones.. N Engl J Med. 1993. PMID 8441427. DOI 10.1056/NEJM199303253281203. (abstract)
Gallagher JC, Smith LM, Yalamanchili V. Incidence of hypercalciuria and hypercalcemia during vitamin D and calcium supplementation in older women.. Menopause. 2014. PMID 24937025. DOI 10.1097/GME.0000000000000270. (abstract)
Hallberg L, Brune M, Erlandsson M, Sandberg AS, Rossander-Hultén L. Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans.. Am J Clin Nutr. 1991. PMID 1984335. DOI 10.1093/ajcn/53.1.112. (abstract)
Jackson RD, LaCroix AZ, Gass M, Wallace RB, Robbins J, Lewis CE, et al.. Calcium plus vitamin D supplementation and the risk of fractures.. N Engl J Med. 2006. PMID 16481635. DOI 10.1056/NEJMoa055218. (abstract)
Massé O, Mercurio CM, Dupuis S, et al.. Calcium, vitamin D, or combined supplementation to prevent fractures and falls: systematic review and meta-analysis.. BMJ. 2026. PMID 42161415. DOI 10.1136/bmj-2025-088050. (abstract)
Myung SK, Kim HB, Lee YJ, Choi YJ, Oh SW. Calcium Supplements and Risk of Cardiovascular Disease: A Meta-Analysis of Clinical Trials.. Nutrients. 2021. PMID 33530332. DOI 10.3390/nu13020368. (abstract)
Singh N, Singh PN, Hershman JM. Effect of calcium carbonate on the absorption of levothyroxine.. JAMA. 2000. PMID 10838651. DOI 10.1001/jama.283.21.2822. (abstract)
Tai V, Leung W, Grey A, Reid IR, Bolland MJ. Calcium intake and bone mineral density: systematic review and meta-analysis.. BMJ. 2015. PMID 26420598. DOI 10.1136/bmj.h4183. (abstract)
Thys-Jacobs S, Starkey P, Bernstein D, Tian J. Calcium carbonate and the premenstrual syndrome: effects on premenstrual and menstrual symptoms.. Am J Obstet Gynecol. 1998. PMID 9731851. DOI 10.1016/s0002-9378(98)70377-1. (abstract)
Wallace RB, Wactawski-Wende J, O'Sullivan MJ, Larson JC, Cochrane B, Gass M. Urinary tract stone occurrence in the Women's Health Initiative (WHI) randomized clinical trial of calcium and vitamin D supplements.. Am J Clin Nutr. 2011. PMID 21525191. DOI 10.3945/ajcn.110.003350. (abstract)
Yang C, Shi X, Xia H, Yang X, Liu H, Pan D. The Evidence and Controversy Between Dietary Calcium Intake and Calcium Supplementation and the Risk of Cardiovascular Disease: A Systematic Review and Meta-Analysis of Cohort Studies and Randomized Controlled Trials.. J Am Coll Nutr. 2020. PMID 31625814. DOI 10.1080/07315724.2019.1649219. (abstract)
. EFSA: Scientific opinion on the tolerable upper intake level of calcium (2012). . . (not-read)
. MedlinePlus: Calcium (health topic). . . (full-text)
. MedlinePlus Medical Encyclopedia: Calcium in diet. . . (full-text)
. MedlinePlus Medical Encyclopedia: Hypercalcemia. . . (full-text)
. NHS: Calcium (vitamins and minerals). . . (full-text)
. NHS: Ciprofloxacin. . . (full-text)
. NHS: Levothyroxine. . . (full-text)
. NIH Office of Dietary Supplements: Calcium fact sheets (health professional and consumer). . . (not-read)
. USDA values summarised in our tofu page (regular tofu set with calcium sulfate). . . (summary-only)

Where to buy

Check the label first. Whatever you buy, look for these:

Both D3 and K2 are listed with the amount of each per serving
Which form of K2 is used (MK-7 or MK-4)
A third-party testing mark (USP, NSF or ConsumerLab) if you want extra assurance

Talk to a pharmacist or doctor before you start any supplement, especially if you take medicines. See the Safety tab for interactions.

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Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.