The classic illegal stimulant for trolls has been Slab, ammonium chloride cut with radium. However, there are a number of other less common and much more dangerous substances. Most, if not all of them, begin with an "s", but there are a few exceptions:
As for why the names of most troll drugs begin with an "s" – it make dem easier to remember.
By the time of Thud!, it becomes apparent that a new drug named Slide has began to replace the dominance of Slab; this causes trolls to go on the rampage and sounds somewhat similar to crack cocaine. The troll in question thinks that he can take on the world. He thinks he has no need for sleep, or food, or drink, and soon enough, he has no need for life. Chrysoprase even claims its introduction has driven him to get out of the drug distribution business altogether.
A similar drug called Crystal Slam is mentioned in Snuff. It's so dangerous that it kills trolls after a few uses. As a result of this, the Diamond King of Trolls has stamped down hard on it's usage: production is illegal throughout the entire world, and it's a hanging offence to try to sell it in any troll city (including, of course, Ankh-Morpork).
Vetinari's position on recreational chemicals for everyone in general and trolls in particular is expressed also in Snuff:
‘You know my position, Drumknott. I have no particular objection to people taking substances that make them feel better, or more contented or, for that matter, see little dancing purple fairies – or even their god if it comes to that. It’s their brain, after all, and society can have no claim on it, providing they’re not operating heavy machinery at the time. However, to sell drugs to trolls that actually make their heads explode is simply murder, the capital crime. I am glad to say that Commander Vimes fully agrees with me on this issue.’