The co-occurrence of substance misuse with other psychiatric disorders is increasingly recognised as a major public health problem. The term ‘dual diagnosis’ has been introduced to describe this phenomenon, but ‘comorbidity’ might be a better term. Community-based surveys in the USA and the UK have reported high rates of comorbidity, particularly in people with serious mental illness (Harrison & Abou-Saleh, 2002). Farrell et al (2001), in a national household survey, reported prevalence rates of psychiatric disorder of 22% in nicotine dependence, 30% in alcohol dependence and 45% in drug dependence, compared with 12% prevalence in the non-dependent population.

The pattern of this comorbidity varies between comorbid mood, anxiety and personality disorders in patients accessing addiction services, and comorbid alcohol, cannabis and cocaine misuse in patients accessing general psychiatric services (Abou-Saleh, 2000). Comorbidity is associated with increased risk of violence, suicide and worse clinical and social outcomes. The National Confidential Inquiry into Suicide and Homicide has reported substance misuse as a factor in over half of homicides and suicides by people with serious mental illness (Department of Health, 2001). Moreover, people with comorbidity have high rates of criminality and blood-borne infections, including HIV infection and hepatitis B and C.

The severity of this morbidity also varies between these specialist settings: severe psychiatric disorder is associated with non-dependent use of substances (problematic substance misuse), whereas severe addiction is associated with personality disorder with or without minor psychiatric disorder.