Black stool after a stomach bug: what to know

Iron tablets, bismuth-containing medicines, and very dark foods can temporarily turn bowel movements black. A bleed higher in the digestive tract can instead produce stool that is black, sticky, tar-like, and unusually foul-smelling. Diarrhoea can make colour harder to judge, but it does not make tarry stool safe to ignore.

Think about anything taken during the illness. Bismuth may darken stool, while iron can make it black or greenish-black. Food-related colour usually fades after the trigger is no longer being consumed. Bleeding is more concerning when the stool looks tarry rather than simply dark, or when the change is accompanied by light-headedness, unusual tiredness, fast breathing, fainting, or worsening abdominal pain.

The original illness may have caused diarrhoea, nausea, fever, cramps, or reduced appetite. Those symptoms can improve while a separate problem becomes apparent, so do not assume every new stool change is part of gastroenteritis. Dehydration can develop after repeated loose stools or vomiting and may add thirst, a dry mouth, or passing very little urine.

Check the stool in good light and note whether it is tar-like, sticky, or mixed with visible red blood. Record when the colour started, how often it has appeared, and whether you used iron, bismuth, aspirin, or other medicines. Do not stop a prescribed medicine without medical advice, but tell a clinician exactly what you took.

Sip oral rehydration fluid or other suitable fluids if you have ongoing diarrhoea or vomiting. Arrange prompt medical advice if black stool persists, returns without an obvious food or medicine explanation, or occurs with weakness or worsening symptoms. A clinician may need to examine you and decide whether blood tests or stool testing are appropriate.

Seek emergency care now for black, tarry stool with fainting, severe dizziness, confusion, marked weakness, cold or clammy skin, vomiting blood or material that resembles coffee grounds, severe abdominal pain, or difficulty breathing. These features can accompany significant bleeding or shock.

Get same-day medical assessment for repeated black stools, black stool with a rapid heartbeat or breathlessness, or signs of dehydration such as very little urine and inability to keep fluids down. Children, older adults, pregnant people, and anyone taking blood thinners should have a lower threshold for contacting a clinician.

Ask whether the stool appearance could be explained by a medicine or food, whether you need testing for blood loss, and how to replace fluids while your bowel symptoms settle. Share the timing of the stomach illness, any vomiting or diarrhoea, your medicines, and whether the stool was sticky or difficult to flush.