For the past 30 years, forensic psychiatry has been concerned with violent offenders with psychosis and/or personality disorder. If dual diagnosis or comorbidity meant anything to a forensic psychiatrist, it would be the ‘typical’ forensic case – an individual with schizophrenia and a premorbid dissocial personality who had been arrested for a violent crime. In this chapter I use comorbidity to describe the co-occurrence of two or more conditions (here a psychiatric disorder and health problems arising from substance misuse) rather than dual diagnosis. In fact, many violent offenders have multiple diagnoses. Williams & Cohen (2000) argue that dual diagnosis suggests a closer relationship, perhaps including cause and effect, and is a subset of comorbidity.

Forensic psychiatrists have probably kept up with developments in criminal law as it relates to crimes associated with alcohol and drugs more than they have with the psychiatry of substance (alcohol and drug) misuse and its relationship to forensic psychiatry. Speciality training arguably encourages this narrow focus. This is unfortunate, as the Epidemiologic Catchment Area (ECA) surveys (Regier et al, 1990) have shown that having a mental disorder doubles the risk of an alcohol misuse disorder and quadruples the risk of a drug-related disorder. The institutional ECA survey of psychiatric patients and prisoners shows even higher levels of comorbidity, with a lifetime prevalence of substance use disorders of 16.7% in the general population and 39% in patients of mental hospitals.