Antibiotics and black stool

Some accompanying products, particularly bismuth used in certain stomach-infection regimens, can harmlessly darken stool. Iron supplements and deeply coloured foods may also change its appearance. Because visual differences can be difficult to judge, new unexplained black stool should not be assumed to come from the antibiotic.

Bleeding that starts in the stomach or upper bowel can be digested before it is passed, producing stool that is jet black, sticky, tar-like, and unusually strong-smelling. A simple dark-brown stool is different, but colour descriptions are subjective and cannot rule bleeding in or out.

Check every medicine and supplement you take, including bismuth, iron, anti-inflammatory pain medicines, and blood thinners. Note when the colour began and whether there is abdominal pain, weakness, dizziness, vomiting, diarrhoea, or visible red blood. This information helps the treating team decide how urgently you need examination and testing.

Contact a healthcare professional promptly for any new black stool that lacks a clear explanation. If you were prescribed a combination pack, read its ingredients and patient leaflet, but do not use that information to dismiss tar-like stool or symptoms of blood loss.

Keep the antibiotic and all other medicine packaging with you, and describe the stool's colour and texture clearly. Do not take an extra antibiotic dose or add an anti-diarrhoeal medicine to treat the colour. A clinician should advise whether prescribed medicines need to be continued, paused, or replaced.

Seek urgent same-day assessment for black, tarry stool even if you do not have pain. Call emergency services if it occurs with fainting, severe dizziness, confusion, shortness of breath, a racing heartbeat, marked weakness, pale or clammy skin, severe abdominal pain, vomiting blood, or vomit resembling coffee grounds.

Frequent watery or bloody diarrhoea with cramps or fever during or after antibiotics also needs prompt evaluation. This pattern can reflect significant bowel inflammation and should not be managed only with over-the-counter remedies.

Ask whether any ingredient in your regimen is known to darken stool, whether your other medicines raise bleeding concerns, and what examination is needed. Confirm exactly which medicines to take while you await assessment rather than making several changes at once.