Well, werechicken, technically speaking you don't need to extract most drugs chemically. I'm not sure how heroine, PCP and crack are made, but they wouldn't really fit the feel of the game anyway. Opium, marihuana and coca (cocaine comes from the coca plant of South America), however, could fit. Opium is just the latex that is gained by making incissions in nearly ripe poppy seedboxes, marihuana is smoked easy as nuthin', and the leaves of the coca plant can be chewed (giving a buzz of energy much like coffee). I'm not saying that just because they're available in the current tech level we have to put them in, but I am a whore for facts and looking smart on the internet (I CAN REMEMBER SIMPLE FACTS, DAMN IT! HONOUR ME AS YOUR NEW GOD!).
Anyway, there was a point here somewhere...oh yeah. Other uses for drugs.
1. Opium or its dorfy equivalent contains morphine, which as you may well know is a painkiller. This could be a new part of the hospital. If we have some kind of pain "stat" (pain is already in the game, but I think it just makes you faint and stuff) whcih directly affects a hospitalized dorf's happyness, painkillers may become important. Just imagine one of your army officers, in traction, delirious with pain...until somebody sticks a pipe stuffed with opium into his mouth, and he becomes normally and properly delirious instead. Too much pain could lead to a few things, like for example serious resentment towards the doctor, or possibly even tantrums, leading to your chief doctor getting a splint stuck through his head.
2. Psychedelic mushrooms or herbs. Hallucinogetic complunds have been used for artists over decades (possibly even centuries, but I can't say I'm an expert on that or anything, so take it with a pinch of salt) for inspiration. Not just writers and painters, but musicians too (let's just say the Beatles enjoyed the occasional chemical-aided relaxation, among many, many others). So let's say Urist McStonecrafter's friends invite him to chew a 'shroom, lick a toad, whatever. Maybe, very rarely, they could go into mini "strange moods" where they make some...experimental art. Like a statue of a dwarf and a unicorn. The dwarf is laughing. The dwarf is riding the unicorn. The statue relates to Urist McStonecrafter's smoking of some gooood shit in the fall of 389. You get the idea, just weird little things, mini-artifacts - not as valuable and not with as many components, possibly even tradable, but just funny shit that you could put in a noble's bedroom or in your meeting area. It could even be really surreal shit, like a statue of a triangle and a square, with the triangle striking down the square...well, I dunno, it's got a pointy end, they could make it work. It could invite some of your more snooty, artsy dwarves (and possibly certain nobles, like the Philosopher if they ever get put back in) to discuss its "post-modern, surrealistic appeal" and its meaning ("The square symbolizes imperialism, obviously, and the triangle represents the right of every dwarf to make his own decisions." "No, no, you got it all wrong, it's..."). High-level craftsdwarves (possibly it'd be masons and carpenters and the like, too) could look at it and either go "hey, that's pretty good, I should make my stuff more like that, it's got a certain appeal..." or "What? This drug-addled rubbish gets praise and wonderment, while my masterpiece figurines of goblins striking down elves get nothing more than a óoh, very nice, well lemme take that to the depot for you'?! CONFOUND YOU, URIST MCSTONECRAFTER! CONFOUND YOU AND YOUR TOAD-LICKING ILK! A CURSE UPON YOUR TAR-STAINED HANDS!".
This could lead to debates between craftsmen - the ones who want to propagate the new experimental style (with a focus on unicorns, odd geometry, images of pipes stuffed with leafy greens or whatever) and those stickler conservatives who want it to be all goblins killing elves, images of cheese in rutile and whatever. Resentment, deep grudges, an furious debates ensue. If ever there's a tantrum spiral, you know the "conservatives"will be going after Urist McStonecrafter - who'll be silently muterring curses to his friend Urist mcStoner as he walls himself in with enough booze to last the year and a few bits of cheese.
3. Hallucinogens applied to bolts and blow darts. Maybe they wouldn't kill goblins, per se, but they'd sit around on the grass, staring at their hands and going "duuuuuude..."".
Just some ideas.