Well, as long as it's a private affair, I have no qualms with people taking illegal drugs. It's up to you how you want to damage your own body and mind, as far as I'm concerned!
It's only when it starts to interfere with how you treat other people that it's a cause for concern... but that responsibility is still that of the person using drugs - they should know better than to do that if it's interfering with their social life and/or work!
(Just like I should know better than to skulk these forums instead of doing study! Luckily I'm on break now though...)