Dried frog pills
As their name suggests, these are pills made chiefly from frogs, specifically the extremely poisonous ones that live in the vivarium at Unseen University and handled by the first-year students, so that if they kill one of them, not too much education has been wasted. Occasionally, one of the frogs is taken to a chamber where it briefly becomes a very happy frog indeed, and then its soul moves on to the great jungle in the sky. Its mortal remains, however, are dried and combined with other ingredients to make small pills.
If just the right combination of ingredients are used, they can make the user hallucinate that they are sane, which is basically what the rest of us do anyway. Their only known user is the university's Bursar, renowned for his degree of insanity. In fact, the phrase "Going completely bursar" has been coined to mean going mental - both in terms of really going mad and, as in the colloquial English, going mad as in losing one's temper violently.
It is interesting in this context that the new Assassins' Guild School has a Tree Frog House. (House master, Mr. Bradlofrudd, serving non-boarding day pupils). The Assassins would also have a keen professional interest in this sort of biology, albeit not for healing or beneficial reasons. A form of frog so virulently poisonous that the merest touch against unprotected skin kills horribly and nearly instantly would of course interest them.
However, in Hogfather we meet Guild Treasurer Mr Winvoe, a man charged with balancing the books down to the last penny. In his brief introduction, the suspicion is there that he is a man doing a very similar job to the Bursar and who shares many of the same character traits - he is a neurovore, albeit one working for a more understanding senior, in the form of Lord Downey. Being Bursar for the Assassins must have its strains upon the nerves, and suddenly discovering an extra three million presumed dollars in the vault, along with the strain of resolving the ambiguity as to whether blank discs made of pure gold are or are not currency ... could it be the case that the Assassins also require a guaranteed in-house supply of Dried Frog produce for their own Bursar? Is this an occupational hazard for those whose world is made up of numbers?
Also in Hogfather, Ridcully typed "D-R-Y-D-F-R-O-R-G-1/2-P-I-L-L-S" and "L-O-T-S-O-F-D-R-Y-D-F-R-O-R-C-P-1/4-L-L-S" into Hex's keyboard, helping Hex recover from meeting the Bursar. It can also be presumed that frog pills are considered an acceptable, if somewhat extreme measure, of treating insanity. Of course, some people are probably beyond all help, as Foul Ole Ron can be considered to be even more insane than the Bursar.