Drug misuse is not a disease, it is a decision, like the decision to step out in front of a moving car. You would call that not a disease but an error in judgement. When a bunch of people begin to do it, it is a social error, a life-style. In this particular life-style the motto is Be happy now because tomorrow you are dying, but the dying begins almost at once, and the happiness is a memory.
Author's Note (pp. 276-277) - A Scanner Darkly (1977)