It's about a girl who's been dabbling with drugs but who becomes a heavy duty user when she starts dating Ledger's character.
I didn't like the film for a number of reasons. First I have a real issue with drugs. Don't get me wrong, I think booze and weed are now culturally acceptable, but I don't like them. The way they affect my sister (who's now clean) didn't help, but I'm fundamentally opposed to them because of the loss of control (and inhibitions) they seem to cause. Get high to your hearts content, just don't do anything after that impacts other people negatively. I don't like some tourists for the same reason. They seem to think it's appropriate to act like Muppets.
Secondly, I don't think the film portrayed drugs negatively enough. Anybody who saw Trainspotting knows that drugs can be unpleasant sometimes, but other than heavy sweating and under eye bags it didn't look too disgusting. I know something sad happened in the film, but that was only indirectly related to the drugs.
Finally their bodies didn't seem to change at all (other than the bags). You hear these stories of weight loss etc, but with them, nothing seemed to happen. I know it's a film and I'm not asking them to do what Christian Bale did in The Machinist (now that was a good film), but the went convincing all the same