So you’re suffering from nausea, heartburn, indigestion, upset stomach, and diarrhea then go ahead and grab your Pepto Bismol. But have you ever noticed that a few hours later your tongue or stool appear black? This happens when the bismuth from the medication reacts with sulphur and saliva in the stomach and form bismuth sulphide. How scientific. Fortunately, the side effect is only temporary and harmless.