Stomach bug and black stool: what to know

Black, sticky or tar-like stool during a stomach bug should be assessed urgently because it may signal bleeding in the upper digestive tract. Iron tablets, bismuth-containing medicines and some dark foods can also change stool colour, but appearance alone cannot safely distinguish these explanations.

Gastroenteritis commonly causes vomiting, diarrhoea, nausea, abdominal pain or fever, but black tarry stool is not a typical feature to dismiss. Blood exposed to digestive juices can turn stool dark and sticky. A clinician may need to ask about stomach pain, vomiting that looks like coffee grounds, medicines, alcohol use and previous digestive problems. A single dark stool after an iron or bismuth product may be harmless, yet ongoing change, weakness or dizziness raises concern for bleeding or dehydration.

Arrange urgent medical advice and show a photo if you can do so privately and safely. Take the packet or a list of all medicines and supplements, including iron, aspirin or anti-inflammatory pain medicines. Continue small, frequent sips of suitable fluids if you can keep them down. Do not take more bismuth, aspirin or anti-inflammatory pain medicine to treat the stomach symptoms unless a clinician tells you to. Avoid alcohol and note any change in stool, vomiting or pain.

Call emergency services or go to an emergency department for black tarry stools with fainting, severe weakness, confusion, fast breathing, severe abdominal pain, vomiting blood or vomit resembling coffee grounds. Urgent assessment is also needed if you cannot keep fluids down, pass very little urine or feel light-headed when standing. These signs can accompany significant bleeding or dehydration. Children, older adults, pregnant people and those taking blood-thinning medicine should seek advice without delay.

Could a medicine or supplement explain the colour, or do I need evaluation for bleeding? Should I stop any current medicine? What symptoms mean I should return immediately? How can I replace fluids safely while nausea or diarrhoea continues?