The short answer
Nausea, stomach pain and constipation are common, especially with older iron salts, and dark green or black stools are normal on iron. Alternate days, bedtime or a gentler form often help. Black, sticky stools with pain or dizziness need a doctor the same day.
| If you get | Try | See a doctor if |
|---|---|---|
| Nausea | Bedtime instead of morning. Alternate days. A gentler form. | It happens with every iron you try |
| Stomach pain or burning | Stop and let it settle. Restart with one product at a time. | It lasts, or you have had ulcers or gastritis |
| Constipation | Alternate days. A chelated form. Water, fibre, movement. | It does not ease within a week or two |
| Dark green or black stools | Nothing: unabsorbed iron colours stool. | Stools are black, sticky and tar-like, with pain or dizziness |
Why it happens
Most of the iron you swallow is never absorbed. It stays in your gut, where it can irritate the lining and shift your gut bacteria. Older iron salts such as ferrous sulfate are the harshest, and a review of trials found they cause significant stomach side effects. Chelated forms, such as iron bisglycinate, are usually gentler.
“The plan is not there for gritting your teeth while you feel very bad. Don't do that.”
Tell your doctor about side effects and ask about the form, the timing or alternate days.
Push through daily nausea or pain.
Same day: black, sticky, tar-like stools with stomach pain or dizziness, black stools when you are not taking iron, or blood in your stool or vomit.
If even a gentle iron always upsets your stomach, do not accept it. Ask your doctor to check your stomach, including for H. pylori and coeliac disease. Some people do better with an iron infusion, which your doctor can discuss.
Questions
Why are my stools black on iron?
Unabsorbed iron turns stool dark green or black. It is common and harmless. Tar-like stools with pain or dizziness are different and need a doctor.
Which iron is gentlest on the stomach?
Chelated forms such as iron bisglycinate are usually better tolerated than ferrous sulfate. Your doctor or pharmacist can suggest one.
Sources
- 1.Tolkien Z. et al. Ferrous sulfate supplementation causes significant gastrointestinal side-effects in adults: a systematic review and meta-analysis. PLoS One, 2015
- 2.NIH Office of Dietary Supplements. Iron: fact sheet for health professionals
- 3.The Iron Guide, part 6: Taking iron well
This page has not been reviewed by a clinician yet. Read it as general education, and take your own results to your doctor.
Educational, not medical advice. ferritin.ai does not diagnose or treat any condition, and it never sets a dose. Talk to your doctor about your own results.