Liam says to Charlie that he, Liam, is Drive Shaft, and that nobody knows who the bass player (Charlie) is. He goes on to tell Charlie that without the band, Charlie is nothing, which spurs Charlie to use heroin for the first time.
When Liam reveals he's Drive Shaft, and no one recognizes Charlie as the bass player, it makes Charlie feel unimportant without the band. Consequently, this realization prompts him to experiment with heroin for the first time.