Mental and behavioural problems due to psychoactive substance use are common. Many psychoactive substances may be problematic including alcohol, opioids, cannabinoids, sedatives, stimulants, hallucinogens, tobacco, and volatile substances. The World Health Organisation (WHO) in the International Classification of Diseases-10 (ICD-10)1 identifies acute intoxication, harmful use, dependence syndrome, withdrawal state, withdrawal state with delirium, psychotic disorder, amnesic syndrome, residual and late-onset psychotic disorder, other mental and behavioural disorders and unspecified mental and behavioural disorders as substance-related disorders. Substance misuse is commonly seen in people with severe mental illness (so-called dual diagnosis) and personality disorder. In many units, dual diagnosis is the norm rather than the exception in adult psychiatry.