Dark stool after antibiotics: what to know

Antibiotics can alter bowel habits and the balance of gut bacteria, leading to diarrhoea, constipation, gas, or a different stool appearance. Iron, bismuth-containing products, dark foods, and some supplements can also darken stool. Colour alone cannot identify the cause. Stool that is truly black, tar-like, sticky, or unusually foul-smelling can indicate bleeding higher in the digestive tract, especially when accompanied by dizziness, weakness, abdominal pain, or vomiting blood. Antibiotics can also be followed by significant diarrhoea that needs review.

Think about when the colour changed and list every medicine, supplement, and strongly coloured food taken recently. Follow the antibiotic instructions and do not share, save, or restart antibiotics without advice. Drink suitable fluids if diarrhoea is present, unless a clinician has restricted your intake. Contact the prescriber if diarrhoea is persistent, severe, or accompanied by cramps or fever. A clinician may examine you and arrange stool or blood tests based on the pattern.

Get urgent medical help for black tarry stool, visible blood, vomiting blood, faintness, severe abdominal pain, confusion, or signs of dehydration such as very little urine. Prompt review is needed if dark stool keeps recurring or you feel increasingly weak. Do not assume the change is a harmless antibiotic effect when warning symptoms are present.

Could another medicine or food explain the colour? Is my stool description concerning for bleeding? Should I change or stop the antibiotic, and what symptoms should I monitor?