drugs.
I've always assumed that they were using heroin in the movie, but I've read some comments that people have posted in which they say that they are using cocaine.
Since watching it again, I noticed how vague they are with the drugs. For instance, they never say heroin, h, smack, or coke (if they indeed are using cocaine); but instead use ambiguous terms like dope and drugs.
Well, I thought it would be clear that they are using heroin since they are called junkies, and also because they need rehab (coke really isn't very hard to quit, at least physically). Plus, they don't seem at all coked out when they take their drugs, just a mellow opiate high.