Soon after waking from a night’s sleep, I clear my throat and spit into the bathroom sink. Whoa! Why is the stuff black? I look at my tongue — it’s black too. What’s going on? I feel fine, but is this a sign of internal bleeding?
I continue with the morning routine, pondering possibilities. I’ve been eating licorice occasionally in recent weeks but not for the past few days, so that’s out. Anything else unusual? Well, I did get up last night and chew a Pepto-Bismol tablet for a slightly queasy stomach. But that was pink, not black. It does contain bismuth subsalicylate, in which bismuth is in the 3+ oxidation state. Could the bismuth have been reduced to elemental bismuth, which is indeed grey? Doesn’t seem likely.
So, I google “black tongue and Pepto-Bismol” and immediately find the answer on the Pepto-Bismol home page. The bismuth can react with sulfur-containing compounds in saliva and form the blackish-brown bismuth sulfide, Bi2S3. Pepto-Bismol says it’s entirely normal and nothing to worry about. I’m relieved. So after decades of familiarity with Pepto-Bismol, I learn something new!
At ChemEd 17 at South Dakota State University, I asked a couple dozen attendees if they had ever experienced a black tongue after taking Pepto-Bismol. Only one knew about it. As an undergraduate he had gone to the doctor, and the doctor said, “Don’t take bismuth!”
Since that occasion, I have tried to repeat the experiment a couple times. The first time, no black tongue. But the second time, I chewed two Pepto-Bismol tablets an hour after eating bacon, sausage and eggs for breakfast, and my tongue was noticeably black a few hours later. It seems likely that the phenomenon would depend on what one has eaten, in order for sulfur compounds to be present in the saliva, and that eggs should be a good source. But it will take much more experimenting.
Readers are invited to experiment!