The environmental risk factors for schizophrenia can be summarized as operating either early in life or later nearer the onset of frank psychosis. This chapter focuses on the role of drug abuse as one of the later factors, and on how it interacts with familial and developmental factors. All the molecular genetic study results discussed are preliminary; both the positive and the negative findings need replication in larger samples. It is likely that certain drugs change the expressions of genes related to neurotransmitter systems such as dopamine or glutamic acid and also for transcription factors, cell proliferation, apoptosis, cell adhesion, and the synapse. Recent research suggests that dopamine sensitization may underlie both craving and the onset of drug-associated psychosis. A drug abuser with low liability to psychosis may use psychostimulant drugs regularly for longer periods without developing psychosis or, at worst may have just brief psychotic symptoms.