Iron Bisglycinate vs Ferrous Sulfate: Fewer Stomach Complaints, but Read the Dose
Ferrous sulfate more than doubles the odds of stomach side effects vs placebo. Bisglycinate trials report fewer complaints, mostly in pregnancy, but trials could compare different doses, which muddies the comparison.
By Kymata Health Editorial Published Last checked
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Key takeaways
From 6 sources- Across 20 placebo-controlled trials (3,168 adults), ferrous sulfate raised the odds of stomach and bowel side effects (OR 2.32)1.
- A 2023 meta-analysis of 17 trials found ferrous bisglycinate gave higher hemoglobin (SMD 0.54) and fewer gut side effects (IRR 0.36) than other iron supplements in pregnant women2.
- In children there was no significant difference in hemoglobin or ferritin, and the 4 trials in other adults were described rather than pooled2.
- In one pregnancy trial, 25 mg of iron as bisglycinate was compared with 50 mg as ferrous sulfate, so fewer complaints may partly reflect the lower dose3.
- Any iron form can reduce absorption of levothyroxine and levodopa, and the adult upper limit is 45 mg a day4.
What this article covers
1 evidence profileThe evidence, graded
Pulled from the trial tables in our evidence profiles. Grades follow our evidence-grading policy.
Graded findings
1 row- AGrade AStomach and bowel side effects with ferrous sulfateWorsened Ferrous sulfate raised the odds of gastrointestinal side effects versus placebo (OR 2.32; 95% CI 1.74-3.08). Meta-regression found no clear link with the iron amount.Iron (ferrous sulfate vs bisglycinate vs other forms) · Meta-analysis · n=3,168 · 2015 · PMID 25700159
The short answer
Ferrous sulfate is the standard oral iron, and it reliably causes more gut side effects than placebo1. Ferrous bisglycinate, an iron amino-acid chelate, is linked to fewer complaints in trials, mainly in pregnant women2.
NIH notes that amino-acid chelates might have fewer gastrointestinal side effects than iron salts4. Whether bisglycinate is better at the same elemental dose is not settled23.
How common the side effects of ferrous sulfate are
A 2015 meta-analysis pooled 43 trials with 6,831 adults1. Against placebo (20 trials, 3,168 people), ferrous sulfate raised the odds of gastrointestinal side effects (OR 2.32, 95% CI 1.74 to 3.08); against intravenous iron the odds ratio was 3.051. In 7 trials in pregnant women (1,028 people) the odds ratio was 3.331.
Meta-regression found no clear link between the dose and the side-effect odds1. NIH, by contrast, says high doses of 45 mg a day or more may cause nausea and constipation, and that taking iron with food can help4.
What the bisglycinate trials show
A 2023 systematic review found 17 randomized trials comparing ferrous bisglycinate with other iron supplements for at least 4 weeks, in any dose or frequency2. In 9 trials in pregnant women, bisglycinate for 4 to 20 weeks gave higher hemoglobin (SMD 0.54, 95% CI 0.15 to 0.94) and fewer gut adverse events (IRR 0.36, 95% CI 0.17 to 0.76)2. Ferritin showed a non-significant trend, there was no significant difference in 4 trials in children, and 4 trials in men and non-pregnant women were only described2.
In a Danish trial, 80 pregnant women took 25 mg of iron as bisglycinate or 50 mg as ferrous sulfate from mid-pregnancy to delivery3. Rates of iron deficiency and anemia were low and similar, and gut complaints were less frequent with bisglycinate (P = 0.001)3. Because the doses differed, the trial cannot separate the effect of the form from the effect of taking half as much iron3.
Reading the label
Iron compounds contain different amounts of elemental iron: ferrous sulfate is 20% iron by weight, ferrous fumarate 33% and ferrous gluconate 12%4. The Supplement Facts panel lists elemental iron, so compare that number rather than the compound weight4.
Many iron-only products provide 65 mg of iron, more than the 45 mg adult upper limit, which is set for healthy people; clinicians sometimes prescribe more to correct a deficiency4.
Who paid for the research
The 2023 bisglycinate meta-analysis was funded by the University of British Columbia, and its authors declared no relevant interests5. The Danish pregnancy trial authors declared no conflicts of interest6.
In the ferrous sulfate side-effect review, two authors noted they had consulted on iron supplementation and are inventors on a patent for a new iron material with potential as a supplement1.
Who should be careful
These cautions apply to every iron form, bisglycinate included4:
- People taking levothyroxine: iron can cut its absorption, and some labels advise against taking levothyroxine within 4 hours of iron4.
- People taking levodopa for Parkinson's disease or restless legs: iron may reduce how much levodopa is absorbed4.
- People on proton pump inhibitors such as omeprazole: lower stomach acid can reduce iron absorption, and people with iron deficiency on these drugs can respond less well to oral iron4.
- People with hereditary hemochromatosis: NIH reports liver specialists advise avoiding iron and vitamin C supplements4.
- Households with children: the FDA-required label warns that accidental overdose of iron-containing products is a leading cause of fatal poisoning in children under 6, to keep iron out of children's reach, and to call a doctor or poison control center immediately after an accidental overdose; at least 43 US children died from swallowing high-dose iron supplements between 1983 and 20004.
- People who also take zinc or calcium: iron at 25 mg or more can reduce zinc absorption, and experts suggest taking calcium and iron at different times4.
Quick answers
Common questions
3 answersIs bisglycinate gentler on the stomach?
Does bisglycinate raise hemoglobin more?
In pregnancy it raised hemoglobin more than other forms; in children there was no significant difference2.
How do I compare doses between forms?
Use the elemental iron on the label; ferrous sulfate is about 20% iron by weight4.
Keep reading
Full evidence profiles
Every graded study, studied doses, interactions and safetyRelated articles
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- ArticleLow Ferritin but “Normal” Blood Count? What Women Should Know About Iron Deficiency Without AnemiaCan you be iron deficient without being anemic?
- ArticleAshwagandha for Stress and Sleep: What the Trials Found, and Why the Results Vary So MuchDoes ashwagandha help with stress and sleep?
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Iron (ferrous sulfate vs bisglycinate vs other forms) products from partner brands
4 products Paid links · we earn a commission- Quick pickHerbsProNOW Foods Iron, 18 mg, 120 Vcaps
1 veg capsule: 18 mg elemental iron (from 90 mg ferrous bisglycinate, Ferrochel)
$8.36one-timeChecked Oct 14, 2026 - Organics OceanBetter Iron
1 capsule: 10 mg iron as SloIron pea ferritin, with 60 mg vitamin C; well below the 60–120 mg iron doses in the trials graded here; 30 servings. Keep iron away from children. An independent lab test (Lead Safe Mama, July 2025) found lead and cadmium in this brand's magnesium glycinate product.
$35.00one-time$29.99subscribeChecked Oct 11, 2026 - JSHealth VitaminsJSHealth Vitamins Iron+ Formula (1-month supply)
1 tablet: 120 mg iron amino acid chelate (Ferrochel; elemental iron per tablet not stated on the product page) with vitamins B1, B2, B6, folinic acid, B12 and 50 mg vitamin C; label says not for use in pregnancy or breastfeeding and carries a California Prop 65 warning. Keep iron away from children.
$19.99one-time$16.99subscribeChecked Oct 15, 2026 - MomentousMomentous Iron+, 60 capsulesWe use this brand at home. What we useSubscribe & save 25%
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1 capsule: 25 mg iron (as ferrous bisglycinate chelate, Ferrochel) with 50 mg vitamin C and B vitamins (B1, B2, niacin, B6, folate, B12, biotin, pantothenic acid); 60 servings. Keep iron away from children.
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References
6 sources. Numbers in the text link here.
Sources cited in this article
6 sources- 1Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysisPLoS One, 2015 · PMID 25700159
- 2The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trialsNutr Rev, 2023 · PMID 36728680
- 3Ferrous bisglycinate 25 mg iron is as effective as ferrous sulfate 50 mg iron in the prophylaxis of iron deficiency and anemia during pregnancy in a randomized trialJ Perinat Med, 2014 · PMID 24152889
- 4Iron: Fact Sheet for Health Professionals (Updated: September 4, 2025)NIH Office of Dietary Supplements, 2025
- 5Ferrous bisglycinate meta-analysis: full text with funding statement (PMC10331582)Nutr Rev, 2023
- 6Ferrous bisglycinate vs ferrous sulfate in pregnancy: article page with conflict-of-interest statementJ Perinat Med (De Gruyter), 2014
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