I'd recommend watching it while take a hit every now and then. Great movie btw. It's a real downer and it's very dark but at the same time it's so fucking true. Quite a few of my closest childhood friends have turned in to really bad junkies and they all talk just like Harry...
It's not really a good movie to watch high because it's got a lot of disturbing and depressing tones to it which will give you a bad high. But if you're going to watch it high, you should smoke first and be sober later.