Stool color is usually explained by diet, but certain shades — especially tarry black or unexplained bright red — can signal a condition that needs medical attention.
Some medicines may turn your bowel movements a lighter shade of brown. If this happens, it is not a serious cause. If you notice a consistent change, you can ask your pharmacist whether this is a known side effect of the medication you are on. However, this change can also be caused by a reduction in bile. The bile may be lacking because it is not being produced properly, or it might be caused by a blockage somewhere along the digestive tract. This can indicate a variety of diseases, including hepatitis and cancer, so be sure to see your doctor if it is not caused by new medication.

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