Magnesium Side Effects, Toxicity and Why Kidney Disease Matters
Magnesium side effects start with diarrhea; high blood magnesium is rare but serious, mostly with kidney failure or heavy laxative and antacid use. Warning signs and upper levels explained.
Content last updated . This page has not been medically reviewed.
Urgent first
Very high blood magnesium can be life-threatening. MedlinePlus says extremely high levels can cause cardiac arrest, and reported symptoms of high magnesium include diarrhea, belly cramping, nausea and fatigue. [1] Case reports and EFSA's review describe low blood pressure, nausea and vomiting, slowed or shallow breathing and, in rare cases, death after large amounts of magnesium-containing laxatives or antacids, and EFSA's review cites historical poisonings from Epsom salt swallowed in large amounts. [2][3][4] Call 911 (US) for collapse, trouble breathing, a very slow heartbeat or confusion after a large amount of a magnesium product. For a child or anyone who swallowed a large amount, call Poison Help at 1-800-222-1222 (US); see Poisoning: what to do first and Poison Help by country.The short answer
Diarrhea is the first and most common sign that supplemental magnesium is more than the gut tolerates. True magnesium toxicity is uncommon because healthy kidneys remove the extra. [5] It becomes a real risk when kidney function is much reduced, or when large or repeated amounts of magnesium laxatives or antacids are used. [5][1]Evidence labels
This page is about safety, so the labels are established nutritional role / deficiency treatment, evidence of harm, mixed and Proper evidence not found. Case reports show that harm can happen; they do not show how often.Common side effects
| Effect | What the sources say |
|---|---|
| Diarrhea and cramping | Sources describe osmotic diarrhea, nausea and abdominal cramping from supplemental (not food) magnesium [6]; the NHS says more than 400 mg a day for a short time can cause diarrhoea [7] |
| Which salts | Easily dissociable salts such as chloride and sulfate, and magnesium oxide, show dose-dependent laxative effects; magnesium naturally in food has not been shown to cause diarrhea in healthy people [2] |
| Long-term high intake | The NHS says there is not enough evidence to say what the effects are of taking high doses for a long time [7] |
Official upper levels (supplement magnesium only)
| Group | Upper level | Source and year |
|---|---|---|
| Children 1 to 3 years | 65 mg a day | Health Canada, page modified 2025 [8] |
| Children 4 to 8 years | 110 mg a day | Health Canada, 2025 [8] |
| Ages 9 and over, including pregnancy and breastfeeding | 350 mg a day | Health Canada, 2025 [8] |
| Adults, pregnancy and breastfeeding, and children from 4 years | 250 mg a day, readily dissociable salts and oxide in supplements, water or added to food | EFSA Scientific Committee on Food, 2006 [2] |
| Infants | Not determined (Health Canada); EFSA could not set a level for ages 1 to 3 | [8][2] |
Health Canada states that its magnesium upper level is for intake from a pharmacological agent only and does not include food and water. [8] EFSA likewise excludes magnesium normally present in foods and beverages. [2] Amounts are mg of elemental magnesium; the levels differ by body, and a UL is not a target.
Why kidney disease matters
Healthy kidneys filter extra magnesium out in urine. [1] MedlinePlus says the most common cause of high blood magnesium is kidney failure. [1] EFSA's review lists severely impaired kidney function as a risk factor, though old age alone was not. [2] Case series describe symptomatic high magnesium from magnesium oxide in older people with reduced kidney function and constipation, one fatal, and a patient with end-stage kidney disease who developed a dangerous heart rhythm after milk of magnesia. [9][4] A fatal case also occurred with normal kidney function, apparently because laxative stayed in the gut and kept being absorbed. [3] A review of magnesium oxide as a laxative says dose, blood levels, interactions and side effects need attention, especially in older adults and people with kidney impairment. [10]People with kidney disease should not start magnesium supplements, laxatives or antacids that contain magnesium without kidney-team advice. See Minerals, supplements and kidney disease.
Other groups
- Medicines: Some medicines raise or lower magnesium or interact with it. [1] See magnesium and medicines.
- Pregnancy and breastfeeding: the Health Canada upper level includes these groups. [8] Ask a clinician before using any supplement.
- Children: keep supplements out of reach; see mineral supplements and child safety.
- Surgery and liver conditions: not described in the sources we read; tell the care team about all supplements.
- Quality: see quality testing and certifications.
Back to Magnesium or the laxative effect of magnesium oxide and citrate.
Frequently asked questions
What are the first signs of too much magnesium?
Diarrhea, belly cramping and nausea are the common early signs; the NHS says more than 400 mg a day for a short time can cause diarrhoea. [1][7]
Can you overdose on magnesium from food?
It is rare to get too much magnesium from food alone. [1] Upper levels apply to supplements and medicines. [8]
Why are people with kidney disease at higher risk?
Kidneys remove extra magnesium; in kidney failure it can build up. [1]
What are the symptoms of dangerously high magnesium?
Reported effects include low blood pressure, nausea, vomiting, slowed breathing, and cardiac arrest at extremely high levels. [2][1] Seek emergency care.
How do I know if my magnesium is high?
Only a blood test can tell; a provider interprets it with your history. [1]
References
- MedlinePlus (US National Library of Medicine). MedlinePlus, Magnesium Blood Test.
- European Food Safety Authority / Scientific Committee on Food. February 2006. Tolerable Upper Intake Levels for Vitamins and Minerals, magnesium chapter.
- Fatal Hypermagnesemia Due to Laxative Use. American Journal of the Medical Sciences 2018. PubMed 29661354. Abstract read 2026-10-23.
- Near death by milk of magnesia. BMJ Case Reports 2017. PubMed 28325719. Abstract read 2026-10-23.
- MedlinePlus (US National Library of Medicine). Review date 1/21/2025. MedlinePlus Medical Encyclopedia, Magnesium in diet.
- US Food and Drug Administration, Center for Food Safety and Applied Nutrition. Letter responding to a qualified health claim petition for magnesium and high blood pressure (Docket FDA-2016-Q-3770); references accessed through December 2021. FDA letter.
- NHS (UK). Page last reviewed 3 August 2020. NHS, Vitamins and minerals: Others (magnesium, phosphorus, potassium, salt).
- Health Canada. Dietary reference intakes, Table 2: reference values for elements. Page date modified 2025-11-19. Health Canada, Reference values for elements. Read 2026-10-23 through a web-fetch tool.
- Severe hypermagnesemia induced by magnesium oxide ingestion: a case series. CEN Case Reports 2019. PubMed 30136128. Abstract read 2026-10-23.
- Magnesium Oxide in Constipation. Nutrients 2021. PubMed 33525523. Abstract read 2026-10-23.
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.