Hello!

Thank you for the query.

Black stool (like a tar) might be a very serious symptom. Usually it indicates presence of blood in the stool. This blood is black because its been digested. So the most common source of such blood is stomach or duodenum. Usually it is a peptic ulcer, however sometimes stomach cancer can occur in such way.
Stomach disease is more probable if you have had some abdominal pains, nausea, loss of appetite, weight loss.

Sometimes such bleeding is caused by a vessel in the small intestine (it is called a malformation).

If you take any iron supplements, black stools are very common side effect of such medicines.

But the bleeding should be ruled out at first. I suggest you to visit your doctor as soon as possible and have blood work done. Gastroscopy should be also performed.

In a meanwhile please avoid coffee and other hot drinks, spicy foods, fruit juices. Lukewarm liquids can be drunk (cold temperature constricts blood vessels).

Hope this will help. Feel free to ask further questions.
Regards.

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