Does iron cause constipation?

What NIH says about iron and constipation

NIH's health professional sheet lists constipation among the effects of high-dose iron: gastric upset, constipation, nausea, abdominal pain, vomiting and diarrhea. Its consumer sheet says the same for healthy people taking high doses, especially on an empty stomach, and adds that doses in the hundreds or thousands of mg can cause far more serious effects.^1,2

High doses of iron supplements, especially on an empty stomach, can cause upset stomach, constipation, nausea, abdominal pain, vomiting and diarrhea. Doses of 45 mg a day or more are the most likely to cause them.^1,2

NIH adds that taking iron supplements with food can help minimize these effects. It also says forms of supplemental iron such as heme iron polypeptides, carbonyl iron, iron amino-acid chelates and polysaccharide-iron complexes might have fewer gastrointestinal side effects than ferrous or ferric salts.¹

What do trials and reviews show?

In 20 placebo-controlled trials with 3,168 adults, ferrous sulfate more than doubled the odds of stomach and bowel side effects (odds ratio 2.32), and the analysis found no clear link with dose.³

That meta-analysis found no clear link between the dose of iron and the chance of side effects, so the analysis could not show that a smaller dose of the same salt would cause fewer effects. The Cochrane review of trials in women who menstruate found the same pattern for constipation specifically.^3,4

In trials in women who menstruate, iron roughly doubled reports of constipation and of loose stools.⁴

In a trial of 150 young women with low iron stores, iron taken on alternate days caused fewer stomach and bowel side effects than the same total amount taken daily, and left fewer women with low iron after 6 months.⁵

Can iron change the color of stool?

The sources checked do not describe stool color changes from iron, which is not a finding either way. A change in stool color, black or tarry stool, or blood in the stool is a reason to ask a doctor, not to assume the supplement is the cause.^1,2

Does iron cause diarrhea, nausea, headaches or weight gain?

NIH names diarrhea, nausea, vomiting and abdominal pain alongside constipation. The sources checked do not report headaches or weight gain from iron taken by mouth, which is not a finding either way.^1,2

Who should ask a doctor about iron and constipation

• Anyone with constipation, stomach pain, vomiting or loose stools that continues after starting iron, who should ask their doctor or pharmacist about the amount, the form and whether to take it with food.^1,2
• Anyone with black or bloody stool, or severe abdominal pain.
• Anyone taking more than 45 mg of iron a day, or an iron product together with a multivitamin.¹
• Anyone who takes levothyroxine, an antibiotic, levodopa or an acid-reducing medicine alongside iron.¹

Questions people ask

Sources

1. Office of Dietary Supplements, National Institutes of Health. Iron: Fact Sheet for Health Professionals. Updated September 4, 2025. Opened 2026-10-07.
2. Office of Dietary Supplements, National Institutes of Health. Iron: Fact Sheet for Consumers. Updated August 17, 2023. Opened 2026-10-07.
3. Tolkien Z, Stecher L, Mander AP, Pereira DI, Powell JJ. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One. 2015;10(2):e0117383. Opened 2026-10-07.
4. Low MS, Speedy J, Styles CE, De-Regil LM, Pasricha SR. Daily iron supplementation for improving anaemia, iron status and health in menstruating women. Cochrane Database Syst Rev. 2016;4:CD009747. Opened 2026-10-07.
5. von Siebenthal HK, Gessler S, Vallelian F, et al. Alternate day versus consecutive day oral iron supplementation in iron-depleted women: a randomized double-blind placebo-controlled study. EClinicalMedicine. 2023;65:102286. Opened 2026-10-07.