yes, it can cause your fecal matter to become dark colored. Diarrhea can be from many other reasons. your stomach might just not like red wine or it is caused by some other thing. Also it is possible you have Inflammatory bowel disease. If so, you should consult a doctor, it is most common in people of 20-25 y. i just read it becouse i was looking an answer similiar to this question.
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