I really wanted to stay out of this, because there's just so many kinds of bias floating around in this thread, clouding the discussion, but I can't resist giving my 2☼.
I think unified mechanics are the way to go with this.
The blood contaminant system that poisons use could be extended to include things like medicines and other drugs. Where the material came from(venom, alchemy, plants, space rocks) and how it got into your bloodstream(smoked/inhaled, eaten, chewed, injected, skin contact) are immaterial, so long as it does something to your dwarf once it's inside him.
The door for this is already open, it's just a matter of Toady coding in more types of effects for blood contaminants, and them linking them to the material raws so that people who want stoner dwarves(pun not intended, but left intact) can put them in.
Mixing dwarven drugs with dwarven medicine would be a great thing for a number of reasons, particularly if the player has to approve any dangerous or untested drugs, before they are administered in his hospital. From injecting Urist CrippledVeteran with random plant extracts, just to see what happens, to arguing with yourself about whether to administer a beneficial, but frighteningly addictive and psychotropic medication, to your champion there's hours of fun and Fun, to be had.
Considering the period knowledge of the long-term, psychological effects of mind altering drugs, it would be cool if not all dwarves were effected the same way. Even the best modern anti-depressents have the opposite effect on a significant portion of the populous. Don't be surprised if the calming drugs you gave to Urist McCrazyPants makes him start chopping people.
Another cool thing would be a drug that can abort a funny mood. The side effects of this would have to be extreme, so only the cruelest or most desperate player would order it done.
Some sort of ETHICS regarding the use of mood altering drugs would be awesome. Goblins and Kobolds would have PERSONAL_CHOICE or ACCEPTABLE , while the other races could run the gamut from PUNISH_SEVERE to JUSTIFIED_IF_NO_REPERCUSSIONS.
In the spirit of keeping DF open and fun for everyone, I think purely recreational drugs (happy thoughts with no significant repercussions) should be kept out of the vanilla raws. This will still leave us with dwarf patients who got hooked on prescription painkillers , and start stealing hospital supplies, which should be enough to satisfy most customers.