What is the theme in Trainspotting by Irvine Welsh?

Drug addiction and social isolation make up the main theme of the book. Early in the novel, Sick Boy states categorically that in the circle with which he and Renton run, there are no friends, only associates. Everybody is looking out for number one, and though temporary entanglements emerge in the common experiences of these junkies, no one is ever more than an associate.