What should you know about constipation cause black stool?

Iron supplements, bismuth-containing medicines and some dark foods can change stool colour. Stool that is truly black, sticky or tar-like, especially with a strong unusual smell, can indicate bleeding higher in the digestive tract. Constipation may occur at the same time, but it does not establish the cause.

Record when the colour changed, its texture, recent foods and all medicines or supplements. Do not rely on an online colour description if you feel unwell; a clinician may need to examine you and arrange tests.

If the stool is black and tarry, contact urgent medical services or a clinician promptly, particularly if this is new or repeated. Bring a list of medicines and supplements, including iron and pain relievers. Do not stop a prescribed medicine without advice, and avoid taking extra anti-inflammatory pain medicine while waiting for guidance.

If a clinician confirms that a supplement explains the colour, discuss constipation separately: regular fluids, gradual fibre and gentle movement may help when suitable. Dehydration can worsen hard stools, but it does not make tarry stool safe to ignore.

Seek emergency care for black tarry stool with dizziness, faintness, weakness, pale or clammy skin, vomiting blood or material resembling coffee grounds, severe abdominal pain, breathlessness or a racing heartbeat. Prompt assessment is also needed for repeated black stools, even if constipation is the main complaint.

Ask whether the appearance could be caused by iron or another product, whether you need tests for bleeding, and which constipation treatment is safe while the stool colour is being assessed.