I think a quest + cash penalty is the best way to approach it, as well as a cooldown on how often you can use the antidote (I suggest a week). If we don't have the time to write a quest, just a cash penalty. Let's say that the antidote works instantly with no penalties, because a SPECIAL penalty would just mean people logoff until it's gone. But to make it a serious decision you can't use drugs for a week - if you do, they kill you.
Making it super complicated to craft just means that people use doctor alts.