Sure, weed or shake-and-bake meth is going to happen, but weed isn't very serious (compared to heroin or cocaine) and small-scale production of hard drugs will yield impure, weak product at a rate that won't support societal usage (and ramping up production requires infrastructure that _would_ get noticed on a small island).

I think you'd be surprised at how much can be produced locally when you don't have competitors with an economy of scale.