"Have you ever seen Requiem For A Dream.... on weeeeeeeed?"
That said.. movies like Trainspotting and Requiem defintely turned me away from trying drugs (mainly umm Heroin).. but then you look at things like Half Baked and Human Traffic that kind of make you say "hey it's cool.. give it a try".
Then there's stuff like Fear and Loathing In Las Vegas that both makes you and not makes you want to try drugs...