Skip to content
AlternativeMed
Iron: Roles, Food Sources, Deficiency, Excess and Supplement Safety

VITAMINS & SUPPLEMENTS · GUIDE

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.

Back to Iron: Roles, Food Sources, Deficiency, Excess and Supplement Safety

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

4 findings · 2 supported · 2 limited or unclearThe colour follows the evidence label on each card.

  1. Ferrous sulfate

    Strong for side effects in adults

    An iron salt, described in that meta-analysis as the current gold standard oral iron therapy. [4]

    What is documented: In 43 trials (6,831 adults) it raised the odds of gastrointestinal side effects versus placebo (odds ratio 2.32). [4]Caution: Side effects are common across iron tablets; see below.

  2. Ferrous bisglycinate

    Moderate in pregnancy; no difference in children

    An amino acid iron chelate. [5]

    What is documented: Fewer GI events and higher hemoglobin in pregnant women than other iron supplements; no significant difference in children. [5]Caution: Trials differ in amounts; see below.

  3. Heme iron

    One trial in a special population

    The iron in animal foods; absorbed through a different pathway than non-heme iron. [6]

    What is documented: In 62 people on peritoneal dialysis, heme iron polypeptide showed no clear benefit over ferrous sulphate and cost 7-fold more. [7]Caution: A kidney-patient trial is not evidence for healthy adults.

  4. Fumarate, gluconate

    Insufficient evidence

    Other forms named in this page's title.

    What is documented: Proper evidence not found in the sources we read for head-to-head trials of these against other forms.Caution: Do not infer from marketing.

What the studies compared

4 findings · 4 limited or unclearThe colour follows the evidence label on each card.

  1. Bisglycinate (25 mg iron) vs sulfate (50 mg iron), pregnancy

    One randomized trial

    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]

  2. Bisglycinate vs other iron, pooled

    Pregnancy: moderate; children: no difference

    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]

  3. Heme iron polypeptide vs ferrous sulphate

    One trial, special population

    Serum ferritin at 6 months was significantly lower in the heme group. [7]

  4. Heme iron, general adults

    Insufficient evidence

    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]

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
References
  1. NHS, Iron (UK). Read 2026-10-23.
  2. 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.
  3. NHS, Iron deficiency anaemia: treatment (UK). Read 2026-10-23.
  4. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One, 2015 Read 2026-10-23.
  5. 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.
  6. Dietary Heme Iron: A Review of Efficacy, Safety and Tolerability. Nutrients, 2025 Read 2026-10-23.
  7. 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.