Why is my poop black after taking vitamins?

The most common reason your poop turns black after taking vitamins is iron. Many multivitamins, prenatal vitamins, and iron supplements contain iron. When some of that iron isn’t absorbed, it can darken stool and make it look dark green or black.

How to check whether iron is the cause

1. Check the label: Look for ferrous sulfate, ferrous gluconate, ferrous fumarate, iron, or carbonyl iron.
2. Think about the timing: If the color change started after you began the vitamin, iron is a likely cause.
3. If you stop the vitamin: Stool color from iron usually returns to normal within a few days, though exact timing can vary.
4. Watch for other symptoms: Iron-related dark stool is often otherwise normal, though it can sometimes cause constipation.

When black stool is more concerning

Black stool can also be a sign of bleeding in the upper gastrointestinal tract (melena). Blood from the stomach or small intestine can turn black and tarry as it is digested.

Get medical help promptly if your stool is:

• Tarry, sticky, or looks like black asphalt
• Very foul-smelling
• Accompanied by stomach pain, vomiting, dizziness, faintness, unusual fatigue, or shortness of breath
• Black even though your vitamins do not contain iron

If you’re unsure, it’s safest to check with a clinician—especially if the black stool is new, keeps happening, or doesn’t improve after stopping an iron-containing vitamin.