Iron (ferrous sulfate vs bisglycinate vs other forms)
Summary
Iron supplements mostly contain ferrous or ferric salts; labels list elemental iron. In menstruating women, daily iron raised hemoglobin and lowered iron deficiency (grade A), but ferrous sulfate roughly doubled stomach and bowel side effects. Alternate-day schedules gave better absorption and fewer side effects in small trials. The adult UL is 45 mg/day.
Key facts
- Ferrous sulfate, gluconate and fumarate; ferric citrate and sulfate; ferrous bisglycinate (amino-acid chelate); carbonyl iron; heme iron polypeptide; polysaccharide-iron complex[1]
- Ferrous fumarate 33%, ferrous sulfate 20%, ferrous gluconate 12% (ODS); ferrous bisglycinate about 27% (PubChem)[1][8]
- Hemoglobin and iron status in menstruating women ↑ (A); low hemoglobin in pregnancy ↓ (B); stomach and bowel side effects with ferrous sulfate ↑ (A)[2][3][4]
- Alternate-day iron was absorbed better than consecutive-day iron and caused fewer side effects in two Swiss trials of iron-depleted women[5][6]
- RDA 8 mg (men, women 51+), 18 mg (women 19–50), 27 mg in pregnancy, 9 mg breastfeeding; 1.8× higher for vegetarians. UL 45 mg/day[1]
- Levodopa and levothyroxine (iron lowers their absorption); proton pump inhibitors may lower iron absorption; calcium may interfere[1]
- 5 graded human outcomes on this page
- October 9, 2026
How it works
EstablishedIron is part of hemoglobin, which carries oxygen from the lungs, and myoglobin, which stores oxygen in muscle; it is also needed for growth, nerve development and hormone synthesis (established). Daily doses raise the iron-regulating hormone hepcidin, which reduces absorption of the next day's dose; this is the proposed basis for alternate-day schedules (proposed).[1][5]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Hemoglobin and iron status in menstruating women | Improved | Grade A | Meta-analysis · n = 6,861 · 2016 51 trials in menstruating women (67 trials, 8506 women overall) Form: daily oral iron, various forms · Dose: Varied across trials · Duration: Varied across trials Hemoglobin was 5.30 g/L higher with iron (high-quality evidence). Iron also lowered the risk of anaemia (RR 0.39; 3273 women) and iron deficiency (RR 0.62; 1088 women), moderate-quality evidence. | 27087396 (opens PubMed)Source [2] |
| Stomach and bowel side effects with ferrous sulfate | Improved | Grade A | Meta-analysis · n = 3,168 · 2015 20 placebo-controlled trials in adults Form: ferrous sulfate · Dose: Varied across trials · Duration: Varied across trials 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. | 25700159 (opens PubMed)Source [3] |
| Low hemoglobin in pregnancy | Worsened | Grade B | Meta-analysis · n = 13,543 · 2024 14 trials in pregnant women (57 trials, 48,971 women overall) Form: daily oral iron, various forms · Dose: Varied across trials · Duration: During pregnancy Daily iron may reduce maternal anaemia (4.0% vs 7.4%; RR 0.30), low-certainty evidence, and probably reduces iron-deficiency anaemia at term (RR 0.41), moderate certainty. Evidence on side effects was very uncertain. | 39145520 (opens PubMed)Source [4] |
| Iron stores and side effects: alternate-day vs daily | Mixed | Grade C | RCT · n = 150 · 2023 Young Swiss women with serum ferritin ≤30 μg/L Form: oral iron · Dose: 100 mg iron daily for 90 days vs on alternate days for 180 days (same total) · Duration: 6 months Ferritin did not differ (43.8 vs 44.8 μg/L). Daily dosing had more gastrointestinal side effects on iron days (LPR 1.56), and iron deficiency at 6 months was 11.4% with daily vs 3.0% with alternate-day dosing. | 38021373 (opens PubMed)Source [6] |
| Iron absorption: alternate-day vs consecutive-day | Improved | Grade D | RCT · n = 40 · 2017 Iron-depleted women aged 18-40 (serum ferritin ≤25 μg/L) Form: ferrous sulfate (isotope-labelled) · Dose: 60 mg iron per morning, consecutive or alternate days · Duration: 14 days (consecutive) vs 28 days (alternate) Cumulative fractional absorption was 21.8% on alternate days vs 16.3% on consecutive days, and total absorbed iron 175.3 vs 131.0 mg. In a second study (n=20), splitting 120 mg into two daily doses did not raise absorption. | 29032957 (opens PubMed)Source [5] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Ferrous sulfate | 20%[1][8] | Ferrous (Fe2+) salts are more soluble and more bioavailable than ferric salts; the reference form in most trials.[1] | About twice the odds of gastrointestinal side effects vs placebo in a meta-analysis of 20 trials.[3] | Most absorption, timing and side-effect trials |
| Ferrous fumarate | 33%[1][8] | No head-to-head human data in our sources.[1] | ODS: high-dose iron salts (45 mg/day or more) may cause nausea and constipation.[1] | Common supplement and prescription form |
| Ferrous gluconate | 12%[1] | No head-to-head human data in our sources.[1] | ODS: high-dose iron salts may cause nausea and constipation.[1] | Common supplement form |
| Ferrous bisglycinate (iron amino-acid chelate) | 27.4%[8] | In a meta-analysis of trials in pregnant women, 4-20 weeks of bisglycinate gave higher hemoglobin than other iron supplements (SMD 0.54); no difference was seen in children. Pooled n not stated in the abstract.[7] | Fewer reported gastrointestinal side effects than other iron supplements in pregnant women (IRR 0.36). ODS notes chelates might have fewer GI effects.[7][1] | Hemoglobin and ferritin in pregnancy and childhood |
| Ferric salts, carbonyl iron, heme iron polypeptide, polysaccharide-iron complex | Varies[1] | Ferric iron is less bioavailable than ferrous iron. No head-to-head data for the others in our sources.[1] | ODS: these forms might have fewer GI side effects than ferrous or ferric salts.[1] | Alternatives for people who do not tolerate iron salts |
- Ferrous fumarate330 mg
- Ferrous bisglycinate (iron amino-acid chelate)274 mg
- Ferrous sulfate200 mg
- Ferrous gluconate120 mg
Compound weight → elemental iron
500 mg ferrous sulfate ≈ 100 mg elemental iron
Studied doses, daily needs and limits
Doses studied in human trials ranged from 60 to 120 mg of elemental iron per dosing day in the timing trials graded here, given daily or on alternate days for 2 weeks to 6 months. These amounts exceed the adult UL of 45 mg/day and were given to iron-depleted women in supervised studies.[5][6][1]
Intake reference: Adult RDA: 8 mg/day for men and for women 51+; 18 mg/day for women 19–50; 27 mg in pregnancy; 9 mg while breastfeeding (ages 19–50). Vegetarians need 1.8 times more. Adult UL: 45 mg/day.[1] The UL is based on gastrointestinal effects. Clinicians sometimes prescribe more than the UL to replenish low iron stores. NIH ODS fact sheet (updated September 4, 2025).
Recommended intake and upper limit by age and sex
| Age | Male | Female | Pregnancy | Lactation | Upper limit (UL) |
|---|---|---|---|---|---|
| 0–6 months (AI) | 0.27 mg | 0.27 mg | — | — | 40 mg |
| 7–12 months | 11 mg | 11 mg | — | — | 40 mg |
| 1–3 years | 7 mg | 7 mg | — | — | 40 mg |
| 4–8 years | 10 mg | 10 mg | — | — | 40 mg |
| 9–13 years | 8 mg | 8 mg | — | — | 40 mg |
| 14–18 years | 11 mg | 15 mg | 27 mg | 10 mg | 45 mg |
| 19–50 years | 8 mg | 18 mg | 27 mg | 9 mg | 45 mg |
| 51+ years | 8 mg | 8 mg | — | — | 45 mg |
Dose math: how much compound supplies that much iron
| Form (elemental %) | 8 mg iron8 mg/day: adult RDA, men and women 51+ | 18 mg iron18 mg/day: adult RDA, women 19–50 | 45 mg iron45 mg/day: adult upper limit (a ceiling, not a target) |
|---|---|---|---|
| Ferrous sulfate (20%) | ≈ 40 mg compound | ≈ 90 mg compound | ≈ 230 mg compound |
| Ferrous fumarate (33%) | ≈ 20 mg compound | ≈ 50 mg compound | ≈ 140 mg compound |
| Ferrous gluconate (12%) | ≈ 70 mg compound | ≈ 150 mg compound | ≈ 380 mg compound |
| Ferrous bisglycinate (iron amino-acid chelate) (27.4%) | ≈ 30 mg compound | ≈ 70 mg compound | ≈ 160 mg compound |
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines
Medicines that can lower levels
- Proton pump inhibitorsNIH fact sheetReduce stomach acid and can reduce nonheme iron absorption; people with iron deficiency on these drugs can respond less to oral iron.[1]
Who should be careful
- Accidental overdose in children
- Iron overdose is dangerous: single intakes above 20 mg/kg can cause severe gut injury, and accidental ingestion of iron supplements caused child deaths in the US. Keep iron products away from children.[1]
- Iron overload conditions
- People with hereditary hemochromatosis absorb too much iron; ODS reports that the American Association for the Study of Liver Diseases advises them to avoid iron and vitamin C supplements.[1]
- Side effects
- High-dose iron can cause stomach upset, constipation, nausea, abdominal pain, vomiting and diarrhea; ODS notes that having iron with food can lessen these.[1][3]
- Testing first
- Low iron has many causes. Iron status should be checked with a clinician before long-term supplementation, especially for men and postmenopausal women, whose needs are low.[1]
- Before you start
- Talk to your pharmacist or clinician before adding this supplement, especially if you take prescription medicines, are pregnant or breastfeeding, or have a chronic health condition.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
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Articles about Iron (ferrous sulfate vs bisglycinate vs other forms)
Plain-English answers to common questions, built on the evidence above.
Frequently asked questions
Is iron bisglycinate gentler than ferrous sulfate?
Some evidence suggests so. A meta-analysis of trials in pregnant women found fewer reported gastrointestinal side effects with ferrous bisglycinate than with other iron supplements, and slightly higher hemoglobin. Data in other groups are limited.[7]
Is every-other-day iron better than daily iron?
In two Swiss trials of iron-depleted women, alternate-day iron was absorbed better (21.8% vs 16.3%) and caused fewer side effects, with similar ferritin at equal total amounts. These are small trials in one population.[5][6]
How much elemental iron is in ferrous sulfate?
About 20% by weight. Ferrous fumarate is about 33% and ferrous gluconate about 12%. US labels list elemental iron.[1]
Why does iron upset the stomach?
Gastrointestinal effects are the most common problem with iron salts. In trials, ferrous sulfate roughly doubled the odds of side effects compared with placebo. ODS notes that having iron with food can help minimize these effects.[3][1]
Can calcium and iron be combined?
Calcium might interfere with iron absorption, although this has not been definitively established. ODS reports that experts suggest using calcium and iron supplements at different times of day.[1]
Sources
- [1]Iron: Fact Sheet for Health Professionals. NIH Office of Dietary Supplements, 2025. fact-sheet
- [2]Daily iron supplementation for improving anaemia, iron status and health in menstruating women.. Cochrane Database Syst Rev, 2016. meta-analysis
- [3]Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis.. PLoS One, 2015. meta-analysis
- [4]Daily oral iron supplementation during pregnancy.. Cochrane Database Syst Rev, 2024. meta-analysis
- [5]Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials.. Lancet Haematol, 2017. RCT
- [6]Alternate day versus consecutive day oral iron supplementation in iron-depleted women: a randomized double-blind placebo-controlled study.. EClinicalMedicine, 2023. RCT
- [7]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. meta-analysis
- [8]Ferrous bisglycinate, ferrous fumarate and ferrous sulfate heptahydrate: molecular formulas and weights. PubChem, NCBI, 2026. database
What changed
- First draft. Every evidence-row PMID checked with NCBI E-utilities (esummary + efetch abstract) and re-read in an independent adversarial pass.
- Added intake table from the NIH ODS fact sheet (checked Oct 9 2026), cost-per-studied-dose method, labeled analysis derived from cited rows.