Do iron supplements cause constipation?

Yes. A Cochrane review of 8 trials in 1,036 menstruating women found that daily iron roughly doubled hard stools or constipation, a risk ratio of 2.07 with an interval from 1.35 to 3.17, and graded that outcome as high quality. Across 27 trials of ferrous sulfate in adults, about 12 in 100 people taking it reported constipation, with an interval from 10 to 15. That figure counts the iron arm only, so some of those people would have been constipated anyway.

Established. Oral iron raises gut side effects. Across 20 randomized trials against placebo, ferrous sulfate more than doubled the odds of them, an odds ratio of 2.32 with an interval from 1.74 to 3.08. Side effects were defined differently from trial to trial.

How much iron raises gut trouble, and what lowers it

Iron against noneOne way of taking iron against another

The top two rows compare iron with no iron, and above 1 means more trouble. The bottom three compare one way of taking iron with another, and below 1 means fewer side effects. The second row is an odds ratio. The heme row is very low certainty. Sources: cards ev-feco-01, -02, -04, -05 and -06 below.

Percent of people with the side effect

The first bar counts constipation among people taking ferrous sulfate, with its interval. It is not a comparison with placebo. The two lower bars come from one small open-label trial whose arms differed in timing with food as well as dose, and its abstract gives no interval. Sources: cards ev-feco-03 and ev-feco-07 below.

What the trials found

How common it is

The Cochrane review covered 67 trials and 8,506 women overall. Besides constipation, daily iron raised loose stools with a risk ratio of 2.13, and any gut side effect with a risk ratio of 1.99 in 5 trials of 521 women. Nausea was not significantly raised. Only 10 of the 67 trials were at low risk of bias, although the constipation result itself was graded high quality.

The ferrous sulfate meta-analysis pooled 43 trials in 6,831 adults. In the iron arms, constipation was the most common gut complaint at 12 percent, close to nausea at 11 percent, ahead of diarrhea at 8 percent. Against iron given by drip, ferrous sulfate tablets raised the odds of gut side effects with an odds ratio of 3.05.

Does the dose matter

Across those 43 trials, the odds of gut side effects did not track the dose in a meta-regression. One small trial points the other way. In 30 iron-deficient women without anemia, 87 percent had gut side effects on 100 mg once a day with a meal, against 7 percent on 6 mg twice a day taken fasting, over 4 weeks. The trial was open label, and its two arms differed in timing as well as dose, so it cannot say which of the two did the work.

Taking it less often

Unsettled. Whether taking iron every other day spares the gut depends on which evidence you look at. In 11 trials in 1,014 people with iron deficiency anemia, side effects were about the same on alternate days as daily, a risk ratio of 1.07 with an interval from 0.86 to 1.34, and the certainty was very low. That review did not report constipation separately. In pregnant women in 22 trials from low and middle income countries, intermittent iron with folic acid caused fewer stomach side effects than daily, a risk ratio of 0.27, at the cost of hemoglobin 0.24 g/dl lower.

Other forms of iron

Across 13 trials, heme iron caused fewer side effects of any kind than ordinary non-heme iron, a risk ratio of 0.62 with an interval from 0.40 to 0.96. The certainty was very low, and most of those trials combined a small dose of heme iron with non-heme iron. In 61 children and teenagers with iron deficiency anemia, adverse events were about as frequent on ferric maltol as on ferrous sulfate over 12 weeks, 29 against 30 percent, while reactions judged at least possibly related to treatment were more common on ferrous sulfate. That trial was open label, stopped early, and did not report constipation separately.

Who should be careful

Not for everyone. If you take levodopa for Parkinson's disease or restless legs syndrome, iron supplements can reduce how much of it you absorb, according to the NIH Office of Dietary Supplements, on evidence from small studies in healthy people. That is a question for whoever prescribes the levodopa before you start iron. This page covers no other medicine, so it is not a full list of what iron interacts with.

In pregnancy, a review of 12 trials found side effects in 21.6 percent of women on iron against 18.0 percent without, a risk ratio of 1.29 with an interval from 0.83 to 2.02, on very low certainty evidence. That difference is not significant. For children and teenagers, the US upper limit for iron ranges from 40 to 45 mg a day by age, against 45 mg for adults, and the limit was set on gut effects. If iron was prescribed for anemia, stopping it because of constipation is worth discussing with the prescriber first, because the form and the schedule are things a prescriber can change.

What expert bodies say

The NIH Office of Dietary Supplements says that high doses of supplemental iron, 45 mg a day or more, may cause gut side effects such as nausea and constipation. It notes that iron-only supplements often provide 65 mg. It says that taking iron with food can help reduce these effects, and food also lowers how much iron is absorbed, a trade-off the fact sheet does not mention in that passage.

How we searched

Searched: a local copy of the PubMed baseline, three times: iron or ferrous salts with constipation or gut side effect terms (233 records, the top 40 read by title), iron with alternate-day, intermittent, lactoferrin or intravenous dosing in meta-analyses and Cochrane reviews (51), and randomized trials of alternate-day iron with gut outcomes (8, one of them retracted). A web search found no meta-analysis newer than the local copy. We also searched the local copy of the NIH fact sheet, drug label interactions and the ClinicalTrials.gov register, which held no trial of iron and constipation. Search run on 6 October 2026.

Included: five meta-analyses, two randomized trials and passages of the NIH fact sheet, plus a review of side effects of iron in pregnancy carried over from our iron pages. None of the 8 study identifiers was found in Retraction Watch.

Excluded: a trial of lactoferrin against ferrous sulfate that is retracted, and a lactoferrin meta-analysis that contains it. A review without pooling, superseded by the 2025 meta-analysis. A Cochrane review of intermittent iron whose side-effect result was not specific to constipation. Reviews of drip against oral iron in bowel and kidney disease. Reviews of liposomal iron and of probiotics with iron, which we did not read in full.

What we read: abstracts, with each quotation checked against the abstract text. The ferrous sulfate meta-analysis was read in full text for its constipation figure.

What we could not get: the live NIH page, which refused our request, so its local copy was used. The full text of the Cochrane review. A 2020 randomized trial of alternate-day iron in The Lancet Haematology, which we could not match to a PubMed record in this run.

What would change this answer

Trials that count constipation as a main outcome with one shared definition. Today it is mostly recorded as a side effect, measured differently in each trial.
A trial of what to do about it. We found none comparing stool softeners or fiber for constipation caused by iron.
Constipation reported separately in the alternate-day trials. The pooled side-effect count cannot show whether taking iron less often helps this one symptom.
Better evidence on heme iron and newer forms. The one pooled result in favor of heme iron is very low certainty.

The rest of the nutrient, in one place. Iron: why the number on the label is not the number your body gets → What it does, how much you need, who runs short, and what too much does, with the evidence level shown on every line.

The full guide

Iron: why the number on the label is not the number your body gets

The same question for other foods (constipation)

Sources

ev-feco-01 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis of RCTs, 67 trials, 8,506 women overall · n = 1,036
These benefits come at the expense of increased gastrointestinal symptomatic side effects.
Who: menstruating women aged 12 to 50 in randomized and quasi-randomized trials of daily oral iron vs control
Effect: hard stools/constipation RR 2.07 (95% CI 1.35 to 3.17, 8 studies); any GI side effect RR 1.99 (1.26 to 3.12, 5 studies, 521 women); loose stools RR 2.13 (1.10 to 4.11); nausea RR 1.19 (0.78 to 1.82, not significant)
Certainty: Constipation outcome graded high quality; overall only 10 of 67 trials at low risk of bias.
Cochrane Database Syst Rev, 2016 · checked 2026-10-06 · we read the abstract
ev-feco-02 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs, random effects · n = 6,831
Our meta-analysis confirms that ferrous sulfate is associated with a significant increase in gastrointestinal-specific side-effects but does not find a relationship with dose.
Who: adults in RCTs comparing ferrous sulfate with placebo (20 trials, 3,168) or intravenous iron (23 trials, 3,663)
Effect: GI side effects OR 2.32 (95% CI 1.74 to 3.08, I2 53.6%) vs placebo; OR 3.05 (2.07 to 4.48) vs IV iron; no significant dose association in meta-regression
Certainty: Odds, not risk; side-effect definitions vary across trials.
PLoS One, 2015 · checked 2026-10-06 · we read the abstract
ev-feco-03 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs, pooled incidence in the active arm · n = 27 studies
For the 27 studies that reported constipation, the pooled estimate of incidence in the FeSO4 arm was 12% [95% CI 10%-15%].
Who: ferrous sulfate arms of RCTs in adults reporting individual gastrointestinal symptoms
Effect: pooled constipation incidence 12% (95% CI 10% to 15%); nausea 11% (8% to 14%, 30 studies); diarrhea 8% (6% to 11%, 25 studies)
Certainty: Incidence in the iron arm only, not a comparison with placebo for this outcome.
PLoS One, 2015 · checked 2026-10-06 · we read the fulltext
ev-feco-04 · Meta-analysis or systematic review · systematic review and meta-analysis of experimental studies, GRADE · n = 22 trials
Intermittent regimens had lower gastric side effects (relative risk (RR), 0.27; 95%CI, 0.11, 0.69) and better medication adherences (relative risk (RR), 1.6; 95%CI, 1.34, 1.91).
Who: pregnant women in low- and middle-income countries, trials of intermittent vs daily iron folic acid
Effect: gastric side effects RR 0.27 (95% CI 0.11 to 0.69); adherence RR 1.6 (1.34 to 1.91); hemoglobin MD -0.24 g/dl (-0.35 to -0.12); anemia RR 1.09 (0.77 to 1.54)
Certainty: Evidence quality ranges from high to very low; LMIC settings only.
Reprod Health, 2025 · checked 2026-10-06 · we read the abstract
ev-feco-05 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs, RoB 2 and GRADE · n = 1,014
Adverse effects were similar between groups (RR: 1.07, 95% CI: 0.86 to 1.34), though metallic taste was more frequent with daily dosing.
Who: people with iron deficiency anemia in the general population, RCTs of daily vs alternate-day oral iron
Effect: adverse effects RR 1.07 (95% CI 0.86 to 1.34); hemoglobin MD 0.28 g/dL favoring daily (-0.01 to 0.56, not significant)
Certainty: Certainty very low for most outcomes; constipation not reported separately in the abstract.
BMC Pharmacol Toxicol, 2025 · checked 2026-10-06 · we read the abstract
ev-feco-06 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs, GRADE · n = 13 studies
Participants receiving HI had a 38% relative risk reduction of total side effects compared to NHI (RR 0.62; 95% CI 0.40; 0.96; CoE: very low).
Who: non-hospitalized groups in RCTs comparing heme with non-heme iron
Effect: total side effects RR 0.62 (95% CI 0.40 to 0.96), certainty of evidence very low
Certainty: Most trials combined low-dose heme with non-heme iron; very low certainty.
Eur J Nutr, 2024 · checked 2026-10-06 · we read the abstract
ev-feco-07 · Randomized controlled trial(s) · randomized open-label study, 4 weeks · n = 30
Gastrointestinal adverse events were reported by 87% of participants receiving standard-dose iron supplementation compared with 7% receiving low-dose iron supplementation (p < 0.0001).
Who: healthy premenopausal women with iron deficiency without anemia
Effect: GI adverse events 87% (100 mg once daily with a meal) vs 7% (6 mg twice daily fasting), p < 0.0001; dose-dependent gut microbiome changes
Certainty: Small and open label; the two arms also differ in timing with food.
Nutrients, 2026 · checked 2026-10-06 · we read the abstract
ev-feco-08 · Randomized controlled trial(s) · phase 3 multicentre randomized open-label trial, NCT05126901 · n = 61
Adverse events were mild/moderate and similar in frequency between the two treatment groups (29% ferric maltol, 30% ferrous sulfate), and adverse reactions deemed as at least possibly related to treatment were more common with ferrous sulfate.
Who: children and adolescents aged 2 to 17 with iron deficiency anemia, 23 sites in the US and UK
Effect: adverse events 29% ferric maltol vs 30% ferrous sulfate, mild or moderate; possibly related reactions more frequent with ferrous sulfate
Certainty: Open label, stopped early at interim analysis; constipation not separated in the abstract.
Paediatr Drugs, 2026 · checked 2026-10-06 · we read the abstract
ev-feco-09 · Position of an expert body · agency fact sheet · n = —
High doses of supplemental iron (45 mg/day or more) may cause gastrointestinal side effects, such as nausea and constipation [5].
Who: users of iron-containing dietary supplements
Effect: threshold 45 mg/day elemental iron; iron-only supplements often provide 65 mg
Certainty: Agency summary; the threshold matches the adult UL.
NIH Office of Dietary Supplements, Iron fact sheet for health professionals · checked 2026-10-06 · we read the section
ev-feco-10 · Position of an expert body · agency fact sheet · n = —
The Tolerable Upper Intake Level for iron is 45 mg for adults, and it ranges from 40 mg to 45 mg for infants, children, and adolescents, depending on age.
Who: adults, infants, children and adolescents
Effect: UL 45 mg/day adults; 40 to 45 mg/day under 19 depending on age
Certainty: Agency position; the UL is set on gastrointestinal effects.
NIH Office of Dietary Supplements, Iron fact sheet for health professionals · checked 2026-10-06 · we read the section
ev-feco-11 · Position of an expert body · agency fact sheet · n = —
Taking iron supplements with food can help minimize these adverse effects.
Who: users of high-dose iron supplements
Effect: advice: take with food to minimize gastric upset, constipation, nausea, abdominal pain
Certainty: Agency advice; food also lowers iron absorption, a trade-off not stated here.
NIH Office of Dietary Supplements, Iron fact sheet for health professionals · checked 2026-10-06 · we read the section
ev-feco-12 · Position of an expert body · agency fact sheet · n = —
Some evidence indicates that in healthy people, iron supplements reduce the absorption of levodopa (found in Sinemet and Stalevo), used to treat Parkinson’s disease and restless leg syndrome, possibly through chelation [101-103].
Who: people taking levodopa
Effect: reduced levodopa absorption, possibly through chelation; levodopa labels carry the warning
Certainty: Agency summary of small studies in healthy people.
NIH Office of Dietary Supplements, Iron fact sheet for health professionals · checked 2026-10-06 · we read the section
iron-p06 · Meta-analysis or systematic review · Cochrane systematic review and meta-analysis · n = 2,423
The evidence is very uncertain for adverse effects (21.6% versus 18.0%; RR 1.29, 95% CI 0.83 to 2.02; 12 trials, 2423 women; very low-certainty evidence)
Who: pregnant women, 12 trials
Effect: adverse effects 21.6 percent against 18.0 percent, risk ratio 1.29 (95% CI 0.83 to 2.02)
Certainty: very low-certainty evidence
Finkelstein et al., Cochrane Database of Systematic Reviews, 2024 · checked 2026-09-16 · we read the abstract

How this page was made. Evidence was extracted from primary sources into cards, every number was re-checked against the source, and the text was written from those cards. Bioma Learn has no human medical reviewer at this time, and we say so rather than invent one. Methodology. This is information, not medical advice.