Black stool after constipation: what to know

Constipation commonly causes hard stools, pushing and discomfort, but it does not normally turn stool black. A dark, sticky or tar-like bowel movement may reflect bleeding higher in the digestive tract. Dehydration can worsen constipation, while hypothyroidism and irritable bowel syndrome can contribute to altered bowel habits in some people. Food, iron tablets and bismuth-containing medicines can also darken stool. Bright red blood on toilet paper after hard stool is a different pattern and can come from the anal area, but any unexplained bleeding deserves attention.

Check whether you recently took iron, bismuth or a very dark food, and record the stool’s colour and texture. Drink fluids unless a clinician has restricted them, and use gentle movement if you feel well. Do not take laxatives to manage unexplained black stool without advice. Contact a healthcare professional promptly if the colour persists, returns, or cannot be explained. Bring a list of medicines and describe your usual bowel pattern, recent straining and any abdominal pain.

Get emergency help for fainting, marked dizziness, unusual weakness, breathlessness, a racing heartbeat, severe abdominal pain, vomiting blood or repeated tar-like stools. These signs may accompany gastrointestinal bleeding or another urgent problem. Seek prompt medical advice even if constipation has improved when stool remains black or you feel unwell.

Could a medicine or food explain this colour? Does the texture suggest digested blood? Should I be checked for dehydration, thyroid problems or another bowel condition? Which constipation treatment is suitable for me?