Stool changes and colon cancer

Possible changes include red or very dark stool, looser stool, constipation, alternating patterns, narrower stool or a feeling that the bowel has not emptied. These changes also occur for many non-cancerous reasons, so the pattern needs medical assessment rather than interpretation by appearance alone.

A tumour may bleed or alter how stool moves through the colon. The exact appearance varies with the site and amount of bleeding, diet, medicines and other bowel conditions.

Bright red blood can come from the lower bowel or anus, while black, tar-like stool may reflect bleeding higher in the digestive tract. Iron tablets, foods and some medicines can also change stool colour. Colour cannot identify the cause on its own.

Note when the change began, whether blood is mixed through the stool or seen on paper, any abdominal pain, weight or appetite change, fatigue, medicines and family history. A photograph can help if describing colour or shape is difficult.

Book a clinical assessment for an unexplained or persistent change. Depending on the history and examination, evaluation may include blood tests, a stool test or colonoscopy. Do not stop prescribed blood-thinning medicine without instructions from its prescriber.

Seek urgent care for heavy or ongoing rectal bleeding, black tar-like stool with weakness or dizziness, fainting, severe abdominal pain, repeated vomiting, a swollen abdomen, or inability to pass stool or gas.

Prompt assessment is also warranted when bleeding accompanies breathlessness, marked paleness or a racing heartbeat. These features can signal significant blood loss or bowel blockage regardless of the underlying cause.

Ask which causes fit your pattern, whether you need a rectal examination, blood count, stool testing or colonoscopy, and how quickly each test should happen.

Check what changes should send you to urgent care and whether any current medicine or supplement could affect bleeding or stool colour.