Heroin Addiction
Is it even possible to do hard drugs and train regularly at the same time? I never did heroin but some friends/former addicts not once told me of how you become indifferent to anything except the drug.
Also if you do not use it regularly all the wonders of withdrawal are your friends. We really do not see this in the movie: Billy gets arrested, uses only sporadically but is able to move around, do things, look after himself and not just lay down cold turkey.