What should you know about black stool after antibiotics?

Antibiotics commonly change bowel habits, but they do not make every black stool harmless. Iron or bismuth taken at the same time, and dark foods, can darken stool. Black, sticky, tar-like stool may instead indicate bleeding in the upper digestive tract. The infection being treated can also cause abdominal upset, while Helicobacter pylori infection is a possible digestive condition that clinicians may consider in the right setting. The antibiotic timing alone cannot establish the cause.

Check the medicine labels for iron or bismuth and record when the colour changed relative to each dose. Note whether stools are loose, tarry, unusually foul-smelling, or accompanied by pain, fever, vomiting, or dizziness. Keep taking an antibiotic exactly as prescribed unless the prescriber advises otherwise, and contact that prescriber promptly about unexplained black stool. Do not use leftover antibiotics or add anti-diarrhoeal medicine without advice.

Seek emergency care for black tarry stool with fainting, severe weakness, confusion, breathlessness, vomiting blood, severe abdominal pain, or inability to keep fluids down. Arrange prompt review for repeated black stools, worsening diarrhoea, dehydration signs, or a new black stool when no dark food, iron, or bismuth explains it.

Ask whether the antibiotic or another product could explain the colour; whether your symptoms suggest bleeding or another bowel problem; and whether you need a medication change, examination, stool testing, or assessment for Helicobacter pylori infection.