The singer describes how the doctor makes regular visits and with equal regularity prescribes Calomel. He comments, "I'm not so fond of Calomel," and asks, "How many patients have you lost? How many patients have you killed Or poisoned with your Calomel?"
Calomel (Hg2Cl2) was one of the first tools in the physician's repertoire that actually did what it was supposed to do. Of course, given what it was used for (a purgative), it is questionable whether it was often needed. In addition, it contains mercury, which is poisonous. - RBW