The short explanation
Dark green or black stools are a very common and expected effect of iron tablets. Unabsorbed iron passes through the gut and colours the stool. On its own, without other symptoms, this is usually not a sign of a problem. Black stool can, however, also come from bleeding higher up in the gut, such as the stomach. That kind is usually tarry, sticky, shiny and foul-smelling, and may come with stomach pain, vomiting, dizziness or fainting. If your stool looks like that, or you feel unwell with it, it needs urgent attention rather than waiting.
What depends on your situation
The parts of this that change from woman to woman. Uncertainty is stated, not hidden.
- If the colour started soon after the tablets and you otherwise feel fine, it is most likely from the iron.
- Painkillers used often, a history of ulcers or acidity, and drinking alcohol make stomach bleeding more possible.
- Tell your doctor about the change at your next visit, especially if you also have stomach pain, constipation or nausea from the tablets, because there are other forms of iron that may suit you better.
What this answer cannot determine
The boundary of online education. These need a clinician who can see you.
- This cannot tell from a description whether your black stool is from iron or from bleeding. A doctor may examine you or test the stool if there is any doubt.
Questions for your clinician
Take these with you. You can add any of them to an appointment card.
- Is this stool colour expected with my tablets?
- What signs would mean it is bleeding rather than iron?
- Is there another form of iron if the side effects bother me?
- Should I avoid painkillers while taking iron?
When to seek help
Clinician-approved guidance for this topic. When in doubt, being seen is never the wrong choice.
Emergency — call 112 now
- Call 112 or go to the emergency department if stools are tarry and sticky with dizziness, fainting, vomiting blood or vomit like coffee grounds.
Get help the same day
- Get care the same day if black stools come with stomach pain, weakness or a racing heart.
Raise it at a routine appointment
- Mention the colour change at your next routine follow-up if you feel well otherwise.
Sources
The evidence this explanation is built on.
- Iron deficiency anaemia NHS
- Iron MedlinePlus (US National Library of Medicine)
- Anemia MedlinePlus (US National Library of Medicine)
Who wrote and reviewed this
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- Author
- AskHer Editorial (AI-assisted draft) AskHer's editorial team — not an individual clinician Works in English, தமிழ், हिन्दी
- Clinical reviewer
- Nobody yet No clinician has signed this answer off, and no person on the editorial team has read it yet. That is what the “Written with AI assistance” label means. It is queued for an editor to read, and then for a clinician who works in this area. The sources it was drafted against are listed above.
- Review status
- Written with AI assistance — not yet reviewed by a person
- Last reviewed
- Not yet reviewed by a person
- Version
- 1
Correction history
Substantive changes are listed, not quietly edited away.
No corrections have been made since this answer was first published.
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