The most common side effects from Pepto-Bismol use for diarrhea include a darker tongue and darker bowel movements, says WebMD. The symptoms most always disappear once use is discontinued.
More serious but infrequent side effects of taking Pepto-Bismol are hearing loss, ringing ears and bleeding from the intestines or stomach, according to WebMD. Other severe -but rare- side effects are vomit resembling coffee grounds, tarry or bloody bowel movements and continual pain in the stomach. Some persons may experience allergic reactions such as tongue swelling, breathing issues and rashes. In such instances, medication use should be discontinued and physician care is recommended, explains WebMD.