Reasons for Dark Stools

Updated November 21, 2016

Dark stools may or may not be serious depending on the cause. Many different foods can actually influence the colour of stool because of excess dyes or colouring. However the Medicine Net website says that dark stools are also caused by underlying medical conditions. See your doctor right away if you are in pain or if your stool colour is consistently dark.

Internal Bleeding

Internal bleeding is a serious cause of dark stools. Very dark, black stools can indicate bleeding in the stomach or small intestine because the blood is older. Bright red blood typically is coming from the colon or anus. Internal bleeding from haemorrhoids is usually not serious, depending on the severity of the problem. The University of Maryland Medical Center says haemorrhoids are a common cause of lower gastrointestinal bleeding. However, internal bleeding from other organs needs to be treated. This bleeding can indicate a number of problems such as an ulcer, hernia or injury. Other causes of internal bleeding include overgrowth of blood vessels or inflammation of the stomach lining.

Iron Supplements

Large doses of iron supplements can also cause dark stools. This can happen when you take too much iron in liquid or pill form. Your body doesn't know what to do with the excess iron so it flushes it out through your stool. Your stools should return to normal within a day or two if you cease taking the supplements.


Foods such as blueberries, dark juices, beets and foods with dyes can also cause dark coloured stools. Colour variations range from dark black to blue or maroon. Black liquorice can cause your stools to look very dark as well. If you consume too many tomatoes your stools can also look bright red or dark maroon. Evaluate what you've been eating and watch to see if your stools maintain dark colouring or return to normal after you stop consuming foods that are causing the dark colour.

Bacterial Gastroenteritis

Bacterial gastroenteritis is an infection that can also cause dark coloured, tarry or red stools. This is caused by inflammation in the digestive tract. The University of Maryland Medical Center says that this condition is caused from eating contaminated food. It can affect one person or an entire group of people who ate at the same establishment. Medical treatment is needed for this condition.

Bowel Tumor

A small bowel tumour can also causes changes in the stool. A tumour is a cancerous or non-cancerous growth that can show up anywhere on the body. A bowel tumour can disrupt the normal functioning of the digestive tract and even cause parts of the bowel to bleed. Sometimes a person with a bowel tumour may experience pain when using the bathroom because the tumour puts pressure on the bowels.

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