Iron Supplement Side Effects: Constipation, Nausea, Dark Stools and When to Seek Care

Seek care first if

• Someone may have swallowed iron, especially a child. In the US call 911 or Poison Help at 1-800-222-1222 without waiting for symptoms. [1] MedlinePlus lists black and possibly bloody stools, diarrhea, nausea, vomiting blood and a metallic taste among the most common symptoms in the first 6 hours. [1] Outside the US see Poison help by country.
• Stools are black and tarry with a foul smell, or you see blood. Iron pills can darken stools, but black or tarry stools can also mean bleeding in the upper digestive tract. MedlinePlus says to contact your provider right away if you notice blood or changes in stool color, vomit blood, or feel dizzy or lightheaded. [2]
• Severe or lasting stomach pain, vomiting or diarrhea. Tell your clinician or pharmacist.

What the sources report

The NHS lists constipation or diarrhoea, tummy pain, heartburn, feeling sick and darker than usual poo as side effects some people get with iron tablets. [3] NHLBI lists a bad metallic taste, vomiting, diarrhea, constipation or upset stomach. [4] MedlinePlus says iron supplements are mostly well tolerated but may cause nausea, vomiting and constipation. [5] The NHS adds that high doses (over 20 mg) of iron can cause constipation, feeling sick, being sick and stomach pain. [6]

Evidence labels used below: established means standard treatment effects reported by health agencies and strong reviews; limited or mixed means small or inconsistent findings.

What trials found

Side effects in studies are averages for groups, not a prediction for one person.

Dark stools: harmless or not?

MedlinePlus lists taking iron pills among the things that can cause black stools, alongside black licorice, blueberries, activated charcoal and bismuth medicines, and says a provider can test the stool with a chemical to check for blood. [2] The NHS lists darker than usual poo as a side effect of iron tablets. [3] The catch is that iron-deficiency anemia can itself be caused by bleeding in the stomach or bowel, so black stool in someone taking iron should not simply be assumed to be the iron. [3] Tell your clinician, especially if you also take NSAIDs (the NHS lists regular NSAID use as a cause of stomach bleeding) [3] or blood thinners (MedlinePlus says anticoagulant and antiplatelet medicines increase the risk of gastrointestinal bleeding). [2]

What the sources suggest if side effects bother you

These are statements from the sources, not a plan for you.

• The NHS says to try taking the tablets with or soon after food to reduce the chance of side effects, and adds that it is important to keep taking them even if you get side effects, and to follow the GP's advice. [3]
• NHLBI says a doctor may suggest taking supplements with food, lowering the dose, or trying a different type of iron supplement. [4] The same page notes that intravenous iron is sometimes used. [4]
• Do not stop, change or add to a prescribed iron regimen on your own. Tell your prescriber how you feel. How timing and dosing patterns affect absorption is on Iron; gaps between iron and other medicines are on Spacing supplements and medicines.

Interactions, pregnancy, children and long-term conditions

• Medicines. The NHS lists calcium and iron supplements among things that may not mix well with levothyroxine, and iron or zinc among those that may not mix well with doxycycline. [9][10] Ask a pharmacist and read How interactions work.
• Pregnancy. A subgroup of trials in pregnant women showed higher odds of stomach effects, so talk to your midwife or doctor. [7]
• Children. Keep iron out of reach. A boxed warning on US solid iron supplements says accidental overdose is a leading cause of fatal poisoning in children under 6. [11]
• Hemochromatosis, kidney or liver disease, surgery. Iron may not be appropriate; see Iron overload and hemochromatosis.