D: Oh no, Yaël! I think I'm having a medical emergency! Quick, call an ambulance!
Y: A medical emergency, Don? Are you sure? You look fine to me.
D: But I'm not. This morning, I had a terrible stomach ache, and now--now--look at my tongue. AAHHHH! It's all black. Surely I'm dying, and you don't even care.
Y: Relax, Don. Of course I care, but you really aren't dying.
D: But my tongue...
Y: I can tell you exactly what the problem is.
D: You can? But you're not a doctor.
Y: But I do read the contraindications before taking medicine.
Y: You said you had a stomach ache, right? And I'll bet you five dollars that you took some Pepto Bismol for it?
D: Wow! You must be psychic. How'd you know?
Y: Well, the active ingredient in Pepto Bismol is a compound called bismuth subsalicylate.
D: Bismuth subsalicylate? So?
Y: This compound has anti-diarrheal, anti-bacterial, and antacid effects on the digestive tract.
D: Which is why Pepto helps with stomach discomfort.
Y: Right. But the thing is, when the bismuth combines with the trace amounts of sulfur in your saliva and in your gastrointestinal tract, it reacts to form a black substance called bismuth sulfide.
D: Bismuth sulfide. Sounds dangerous.
Y: It isn't. But that's where the discoloration on your tongue comes from. And bismuth sulfide can also temporarily darken the stool. It's a common enough side-effect that it's listed in the package information, which you really should get in the habit of reading. In a couple of days, everything will be back to normal.
D: Phew, Yaël. You saved my life!Y: Not really.