TD12 is a drug mostly used by dentists in England. It’s an anesthetic and sedative, which has the side effect of affecting memory formation.
This side effect makes it very interesting to a certain kind of criminal – notably, the murderer Culverton Smith once dosed some of his nearest and dearest with it so that he could tell them of his planned murder without them remembering it. Fortunately, Sherlock Holmes deduced what he had done, and Smith paid the price for his crimes.