A high proportion of people who have a serious mental illness also use non-prescribed psychoactive drugs. This association is perhaps most apparent, and possibly also has the greatest implications for health, in relation to tobacco use. In fact recognition of the special relationship between mental illness and smoking was recently reflected in exemption of psychiatric units from legislation to ban smoking in enclosed public spaces in the UK (UK Smoking Law Resource Centre, 2007). The prevalence of tobacco use has generally decreased in most European, North American and Australasian countries, however, use of other psychoactive drugs, has not followed this pattern (Advisory Council on the Misuse of Drugs, 2006). Cannabis use in particular is widespread, both in the general population but especially so amongst people with serious and enduring mental illness, particularly psychotic illness. Recognition of the particular coincidence of cannabis use with psychotic illness amongst psychiatric patients was one of the observations that precipitated current debate around the possible aetiological role of cannabis use in relation to incidence of schizophrenia (Thornicroft, 1990; Hall, 2006; Macleod et al. 2006).