Ferrous Sulfate vs Iron Bisglycinate: Gentler on Your Stomach?
Yes, bisglycinate is usually gentler. In trials of pregnant women it caused 64% fewer stomach side effects than other iron. But guidelines say no form works better, and ferrous sulfate costs about 12 times less for the same iron. Start with ferrous sulfate, every other day if it bothers you; switch if it still does.
By Erin Rose Updated October 6, 2026 How we score
How much to take: our iron dosage guide.
Side by side
| Form | Iron by weight | Stomach | What trials found |
|---|---|---|---|
| Ferrous sulfate | 20% (a 325 mg tablet gives 65 mg of iron) | More than doubled the odds of stomach side effects in 20 trials against a placebo (Tolkien 2015). | The standard first treatment in UK guidelines: one tablet a day (BSG 2021). Doctors’ guidelines say no oral form works better than another (AGA 2024). |
| Iron bisglycinate (ferrous bisglycinate) | Not fixed; the label states the iron (18 to 36 mg a capsule in the products we score) | Fewer stomach side effects than other iron in trials of pregnant women: 64% fewer reported (Fischer 2023). | Higher hemoglobin than other iron in pregnant women; no difference in children; too few trials in other adults to pool (Fischer 2023). 25 mg kept iron up as well as 50 mg of ferrous sulfate in pregnancy (Milman 2014). |
Which is gentler
Bisglycinate, in the trials we have. A 2023 review of trials comparing it with other iron found 64% fewer reported stomach side effects in pregnant women (Fischer 2023). In one trial, 25 mg of bisglycinate caused fewer stomach complaints than 50 mg of ferrous sulfate and kept iron up just as well (Milman 2014). For ferrous sulfate, 20 placebo-controlled trials found it more than doubled the odds of stomach side effects such as nausea and constipation (Tolkien 2015).
Doses differ too: the bisglycinate capsules on sale hold 18 to 36 mg of iron, the standard sulfate tablet 65 mg, and the trials did not compare them at the same dose. Before you switch, try ferrous sulfate every other day: UK guidelines suggest it when a daily tablet is not tolerated (BSG 2021), and in a trial it caused fewer stomach side effects with the same gain in iron stores (von Siebenthal 2023).
Which works better
Neither, as far as guidelines go: “No single formulation of oral iron has any advantages over any other” (AGA 2024). The review above found bisglycinate raised hemoglobin more in pregnant women, made no difference in children, and had too few trials in other adults to say (Fischer 2023). In 200 Mexican schoolchildren with low iron stores, 30 mg a day of either form raised ferritin, and the gain lasted six months (Duque 2014).
Ferrous sulfate is what UK guidelines start with: one tablet a day of ferrous sulfate, fumarate or gluconate (BSG 2021).
| Item | Value |
|---|---|
| Nature Made Iron 65 mg | $0.054 |
| Piping Rock Iron 65 mg | $0.064 |
| Piping Rock Slow Release Iron 45 mg | $0.087 |
| Thorne Iron Bisglycinate 25 mg | $0.64 |
| Thorne Advanced Iron Complex 36 mg | $0.72 |
| Momentous Iron+ 25 mg | $0.80 |
On Amazon: Nature Made Iron 65 mg · Piping Rock Iron 65 mg · Piping Rock Slow Release Iron 45 mg · Thorne Iron Bisglycinate 25 mg · Thorne Advanced Iron Complex 36 mg · Momentous Iron+ 25 mg
On the testing lists, 60 mg of iron costs $0.054 as ferrous sulfate and $0.64 as bisglycinate, the cheapest of each. Every product we score.
Other forms on the shelf
| Form | Iron by weight | What we know |
|---|---|---|
| Ferrous fumarate | 33% | NIH groups it with the other ferrous salts. One of the three tablets UK guidelines start with (BSG 2021). |
| Ferrous gluconate | 12% | NIH groups it with the other ferrous salts. One of the three tablets UK guidelines start with (BSG 2021). |
| Carbonyl iron, heme iron, polysaccharide-iron complex | Varies; the label states the iron | NIH says these, and amino-acid chelates, “might have fewer gastrointestinal side effects than ferrous or ferric salts.” |
You do not need the percentages: the “Iron” line on the Supplement Facts panel already gives the iron itself.
Questions
Is iron bisglycinate better absorbed than ferrous sulfate?
Not proven for most people. In pregnant women, bisglycinate raised hemoglobin more than other iron; in children it made no difference, and there are too few trials in other adults to say (Fischer 2023). Doctors’ guidelines say no oral form works better than another (AGA 2024).
Does bisglycinate cause constipation?
Less often, in the trials we have, mostly in pregnant women (Fischer 2023; see above). Ferrous sulfate more than doubled the odds of stomach side effects in 20 trials against a placebo (Tolkien 2015).
Is 25 mg of bisglycinate the same as 65 mg of ferrous sulfate?
No. 65 mg is the iron in one 325 mg ferrous sulfate tablet; 25 mg is less than half that. In pregnancy, 25 mg of bisglycinate kept iron up as well as 50 mg of ferrous sulfate (Milman 2014), but that was prevention in women who were not yet low.
Can I take ferrous sulfate every other day instead?
Yes. UK guidelines suggest one tablet every other day when a daily tablet is not tolerated (BSG 2021). In a trial in iron-depleted women, every-other-day iron gave the same gain in iron stores with fewer stomach side effects (von Siebenthal 2023).
Sources
- Fischer JAJ, et al. The effects of oral ferrous bisglycinate supplementation on hemoglobin and ferritin concentrations in adults and children: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev. 2023. PubMed 36728680
- Tolkien Z, et al. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults. PLoS One. 2015. PubMed 25700159
- Milman N, et al. 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. PubMed 24152889
- Duque X, et al. Effect of supplementation with ferrous sulfate or iron bis-glycinate chelate on ferritin concentration in Mexican schoolchildren. Nutr J. 2014. PubMed 25023784
- DeLoughery TG, et al. AGA Clinical Practice Update on Management of Iron Deficiency Anemia. Clin Gastroenterol Hepatol. 2024. PubMed 38864796
- Snook J, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut. 2021. PubMed 34497146
- von Siebenthal HK, et al. Alternate day versus consecutive day oral iron supplementation in iron-depleted women. EClinicalMedicine. 2023. PubMed 38021373
- NIH Office of Dietary Supplements. Iron fact sheet for health professionals (updated September 4, 2025; read October 6, 2026).