Short answer
Yes. Iron supplements, and multivitamins and prenatal vitamins that contain iron, commonly turn stool dark green or black. This is a known and usually harmless effect. Stool that is black, sticky and tar-like with a strong smell, or comes with dizziness or belly pain, should not be blamed on iron. Get it checked.
Why does iron darken stool?
Iron can darken stool because your body absorbs only part of the iron you take. The rest passes through the gut and reacts with compounds there, turning stool dark green, dark brown or black.
How can you tell iron stool from bleeding?
Stool darkened by iron is usually dark green or black but formed, while black stool from bleeding is typically sticky, tar-like and foul-smelling.
| Iron | Bleeding (melena) | |
|---|---|---|
| Color and texture | Dark green to black, usually formed | Black, sticky, tar-like, shiny |
| Smell | Usual | Unusually foul |
| How you feel | Well, or mild stomach upset | Dizzy, weak, pale, or with pain or vomiting blood |
What other side effects can iron supplements cause?
Iron can cause constipation, nausea and stomach upset. If side effects bother you, ask your doctor or pharmacist. Do not stop or change a prescribed supplement without talking to them.
Iron is dangerous for children. Accidental swallowing of iron supplements and prenatal vitamins can cause serious poisoning in young children. Keep them out of reach. If a child swallows iron tablets, call poison control right away. In the US, that is 1-800-222-1222.
When should you see a doctor about black stool on iron?
Contact a doctor if stool stays black for more than a few days after stopping iron, or if your stool is black and you are not taking iron. Black, tarry stool with dizziness needs care right away.
Expected
- Dark green or black, formed stool while you take iron, with no other symptoms
Call a doctor
- Stool stays black for more than a few days after stopping iron
- You are not taking iron and stool is black
- Severe constipation or stomach pain from iron
Get care now
- Black, sticky, tar-like stool
- Dizziness, fainting, or vomiting blood
Warning signs to know
These are signs that published medical guidance says need prompt or urgent care. The list is not complete, and ticking boxes cannot tell you whether you are ill. If you are worried, contact a clinician whatever you tick.
Questions people ask
Does iron make stool black?
Yes, often. It can also make stool dark green. It is usually harmless, but ask your doctor if you are unsure.
Do iron tablets make stool green?
Yes. Iron can give dark green or black stool.
How can I tell iron stool from blood?
Iron stool is usually dark and formed. Stool from bleeding is sticky, tar-like and foul-smelling, and you often feel unwell.
Is it normal for a pregnant woman on iron to have black stool?
Yes, dark stool is common with prenatal iron. Mention any black, sticky stool or dizziness to your provider.
Sources
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