I say with confidence, that if somebody is HEAVY in to drugs and not just taking some random ski trips, it will like impact the session and certainly increase the odds that things don't go as planned...
To relate it to the real world....I own a business. I know that addicts can typically be very hard workers for a buck, but inconsistency, erratic actions, and presentation all stop me from ever having any around. Where as, the guy who likes to let loose a little from Friday night to Sunday morning, I just don't depend on during those times....Lol