A drug used by Trolls, consisting of ammonium chloride cut with radium. A growing problem in Ankh-Morpork, because of the number of troll boys using it. Detritus has a One-Step program to help addicts. As of Feet of Clay there is also a one step program for the Dealers, since while Detritus is very careful to lie to Vimes about it, we are clearly informed that he nailed a dealer's ears to the wall. Detritus himself has had a Slab awareness campaign going since Feet of Clay, telling people what happens to "them buggers what sells Slab to kids".
Slab: Jus' say 'AarrghaarrghpleeassennononoUGH'.
Commander Vimes apparently authorised this campaign.
The actual effects of Slab are similar to LSD with the Troll in question seeing things for a prolonged period, not causing any trouble, and just wandering off to look at the pretty pictures. While the Troll will usually go find somewhere quiet to enjoy the show, it has nasty side effects up to and including melting a trolls brain.
Chrysoprase is a suspected pipeline dealer in Slab, though as of Thud! he is attempting to claim he's getting out of the trade owing to the influx of young Trolls on Slide. During the book he even gives up the location of a drug lab to the watch though given that Mr Chrysoprase is a mobster the truth (or lack of) of his statement cannot be determined.
Detritus's campaign is a parody of the "Just Say No" campaign, started by the BBC children's programme Grange Hill after a storyline in which pupil Samuel "Zammo" McGuire battled heroin addiction.