Most of the drugs available in the 14th century and before will be of the entirely natural, unprocessed variety.
The only ones that really seems like a danger, as far as mass abuse is concerned, are opium, and possibly hashish, both of which would require processing, which would atleast limit their availability.
In general, the more severe and/or toxic, like ergot, toadstools, toads, etc. don't seem to historically have been abused, on anywhere near the same levels as more modern drugs.
Not like crack, heroin, meth, etc.
Ergot pollution for instance might cause you occasional problems as a player, but it's not something you'd be actively able to inflict on your neighbors, or really want to, for that matter.
You might feed some to your goblin prisoners, or whatever, but there's really no guarantee that it's going to drive them insane, in any entertaining fashion, anyway. And certainly not on a permanent basis.
Drugs back then were mostly used-when they were used at all-either medicinally, or ritually. Possibly sometimes to strengthen your own soldiers (Norse berserkers, Zulu warriors, etc), but not really in a pharmaceutical or mercantile fashion.
Even opium and hashish, the worst of the lot, had beneficial side-effects.
Ergot, magic mushrooms, and the like, might be used perhaps to poison an invading army, but there again, it's probably something you want to keep the secret of to yourself, rather than giving or selling it to your enemies.