Black poop after antibiotics: what to know

Antibiotics can change bowel habits and gut bacteria, but black stool is not a typical explanation to dismiss automatically. Iron, bismuth-containing remedies and very dark foods can change stool colour. Bleeding higher in the digestive tract can make stool black and tar-like. The antibiotic may have been prescribed for conditions such as [Acute Bronchitis](/conditions/acute-bronchitis/), [Cellulitis](/conditions/cellulitis/) or [Helicobacter Pylori Infection](/conditions/helicobacter-pylori-infection/), but the original illness does not identify the cause of the colour change.

Check the medicine list, including iron, bismuth and supplements, and note when the colour changed. Do not take extra doses or stop an antibiotic early unless the prescriber advises it. Drink fluids if you have loose stools, and avoid anti-inflammatory painkillers until a clinician has considered bleeding. Arrange prompt medical advice if the stool remains black or you cannot explain the change.

Seek emergency help now for black tarry stools with dizziness, fainting, severe weakness, breathlessness, vomiting blood, or worsening abdominal pain. Get same-day advice if you have repeated black stools, a fast heartbeat, pale skin or ongoing vomiting. Antibiotic-associated diarrhoea with fever or dehydration also needs prompt review.

Could any of my medicines explain the colour? Does the stool look like melaena rather than simply dark brown? Do I need blood tests, stool testing or other assessment, and should my antibiotic be changed?