Iron Forms Compared: Ferrous Sulfate, Fumarate, Gluconate, Bisglycinate and Heme
Iron forms compared (ferrous sulfate, bisglycinate, heme iron and others): what trials show about stomach side effects and blood counts, child safety, and why a blood test comes first.
Content last updated . This page has not been medically reviewed.
Urgent first: iron and children
Iron supplements can be dangerous to small children if swallowed. The NHS says very high doses of iron can be fatal, particularly if taken by children, so keep iron supplements out of their reach. [1] If a child may have swallowed iron, treat it as an emergency: in the US call 911 or Poison Help at 1-800-222-1222, and see Poisoning: what to do first, Poison help by country and Iron overdose and child safety.The question
Is a gentler iron form such as ferrous bisglycinate as good as ferrous sulfate, and what about heme iron? In trials that compared forms, ferrous bisglycinate was not inferior to ferrous sulfate in pregnancy and caused fewer stomach complaints. [2] Heme iron has less direct evidence. There is no universal winner. Do not choose a form to treat suspected low iron on your own: tiredness has many causes, and the NHS says the GP will usually do a blood test to find out if the number of red blood cells is normal. [3] See Iron for the full profile.The forms side by side
| Option | What it is | What is documented | Evidence strength | Caution |
|---|---|---|---|---|
| Ferrous sulfate | An iron salt, described in that meta-analysis as the current gold standard oral iron therapy. [4] | In 43 trials (6,831 adults) it raised the odds of gastrointestinal side effects versus placebo (odds ratio 2.32). [4] | Strong for side effects in adults | Side effects are common across iron tablets; see below. |
| Ferrous bisglycinate | An amino acid iron chelate. [5] | Fewer GI events and higher hemoglobin in pregnant women than other iron supplements; no significant difference in children. [5] | Moderate in pregnancy; no difference in children | Trials differ in amounts; see below. |
| Heme iron | The iron in animal foods; absorbed through a different pathway than non-heme iron. [6] | In 62 people on peritoneal dialysis, heme iron polypeptide showed no clear benefit over ferrous sulphate and cost 7-fold more. [7] | One trial in a special population | A kidney-patient trial is not evidence for healthy adults. |
| Fumarate, gluconate | Other forms named in this page's title. | Proper evidence not found in the sources we read for head-to-head trials of these against other forms. | Insufficient evidence | Do not infer from marketing. |
What the studies compared
| Forms | What the source found | Strength of evidence |
|---|---|---|
| Bisglycinate (25 mg iron) vs sulfate (50 mg iron), pregnancy | In 80 healthy Danish pregnant women, bisglycinate was not inferior to sulfate, and GI complaints were lower in the bisglycinate group (P=0.001). [2] | One randomized trial |
| Bisglycinate vs other iron, pooled | A meta-analysis of 17 trials found higher hemoglobin and fewer GI adverse events in pregnant women; no significant differences in children; more trials are needed in other groups. [5] | Pregnancy: moderate; children: no difference |
| Heme iron polypeptide vs ferrous sulphate | Serum ferritin at 6 months was significantly lower in the heme group. [7] | One trial, special population |
| Heme iron, general adults | Proper evidence not found in the sources we read (PubMed records, 2026-10-23). A 2025 review of heme iron declares that three of its authors were employed by a company. [6] | Insufficient evidence |
Reading the table carefully
- Different amounts. The pregnancy trial compared 25 mg of iron as bisglycinate with 50 mg as sulfate, so it tested a lower amount of one form against a higher amount of the other. [2]
- Population matters. The pooled advantage of bisglycinate was in pregnant women, with no difference in children. [5]
- Heme iron. It comes mainly from animal foods. [6] The review's own conflict-of-interest note matters when weighing its enthusiasm.
No universal winner
Bisglycinate has pregnancy trial data showing fewer stomach complaints, sulfate has the most side-effect data, and heme iron lacks general-adult trials in the sources we read. The choice belongs with a clinician after a blood test.Frequently asked questions
Is iron bisglycinate better than ferrous sulfate?
In pregnancy trials it was not inferior and caused fewer stomach complaints; no difference was seen in children. [2][5]
Does ferrous sulfate cause more side effects?
Compared with placebo, yes, in a meta-analysis of 43 trials. [4]
Is heme iron better absorbed?
Heme iron is absorbed through a different pathway. [6] Proper evidence not found for a better result in general adults.
Should I take iron without a blood test?
The NHS describes diagnosis by blood test. [3] Talk to a clinician first.
What if a child swallows iron tablets?
Treat it as an emergency; in the US call 911 or Poison Help at 1-800-222-1222. [1]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
How to read the evidence labels
Labels used on this page: evidence in specific populations, mixed, insufficient evidence.
Official amounts, side effects and practical points (UK, NHS)
- UK reference amounts: 8.7 mg a day for men aged 19 and over, and 14.8 mg for women aged 19 to 49. [1] The NHS says 17 mg or less a day of iron supplements is unlikely to cause harm, and that high doses (over 20 mg) can cause constipation, feeling sick, being sick and stomach pain. [1] It also says to continue a higher dose if a GP advised it. [1]
- Common iron-tablet side effects include constipation or diarrhoea, tummy pain, heartburn and darker than usual poo. [3]
- The NHS suggests trying iron tablets with or soon after food to reduce side effects, and says tea, coffee, milk and dairy, and foods high in phytic acid can make it harder to absorb iron. [3]
- We could not read the US NIH Office of Dietary Supplements page (access blocked), so we quote no US figures; see RDA, AI, DV and UL.
Safety and who should ask a clinician
- Do not treat suspected low iron alone. A clinician decides after testing. See Deficiency testing and its limits.
- Pregnancy and heavy periods. The NHS says heavy periods and pregnancy are very common causes of iron deficiency anaemia. [3] See Women's nutrition: iron, folate and calcium and Pregnancy and breastfeeding.
- Medicines and minerals. See Mineral interactions and absorption, Spacing supplements and medicines and How interactions work.
- Liver, kidney, surgery. Ask a clinician; see Supplements before surgery.
References
- NHS, Iron (UK). Read 2026-10-23.
- Ferrous bisglycinate 25 mg iron is as effective as ferrous sulfate 50 mg iron in the prophylaxis of iron deficiency and anemia during pregnancy. J Perinat Med, 2014 Read 2026-10-23.
- NHS, Iron deficiency anaemia: treatment (UK). Read 2026-10-23.
- Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One, 2015 Read 2026-10-23.
- The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis. Nutr Rev, 2023 Read 2026-10-23.
- Dietary Heme Iron: A Review of Efficacy, Safety and Tolerability. Nutrients, 2025 Read 2026-10-23.
- A randomized controlled trial of oral heme iron polypeptide versus oral iron supplementation for the treatment of anaemia in peritoneal dialysis patients: HEMATOCRIT trial. Nephrol Dial Transplant, 2012 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.