definitely drugs as some pl have already figured out just to verify at the end of the hospital ending he got a overdose but without injecting nerves or arteries in one ending he was able to just walk away and leave his past life behind and in the last ending he decided to just stay on drugs for the rest of his life but being careful about how much he took my guess is that the girl Leigh is heroin or crack/cocaine or some other type of drug that's injected