I'm trying; I really am!
It's just so hard to stay serious about this, since drugs in rougelikes are usually silly gags, like the Nethack hallucinogen.
Maybe drug farming should be much more dependent on local climates, since it isn't so necessary for survival as food. Eventually, things like greenhouses could compensate for this, but until then, import, or build a city in a place that can grow it.
If you don't want to pay the overinflated price for elven Pipe Weed, you have to grow your own.
With supply and demand, a player could build a large farming center for drugs, before building his other cities, to ensure they have a cheap supply for the herbally minded members of the nobility. This applies to anything, but if suitable locations are hard to find, the player would have to be more deliberate about it for drugs.
Also, should there be some reflection of the dangers of mixing drugs, especially stimulants, with alcohol? It's a lot easier to OD on just about anything with alcohol in your system, and mixing stims with depressants (including alcohol) can be dangerous, particularly for those at risk for heart attacks.