Dual diagnosis is the term used to describe people with mental health and substance abuse problems.

The most recent survey of mental illness and substance misuse amongst prisoners, and also the most comprehensive, was reported in a 1997 survey carried out by the Office for National Statistics. The survey found that 72% of adult male and 71% of female prisoners may have two or more mental disorders (including personality disorder, psychosis, neurosis, alcohol misuse and drug dependence), while 20% have four or more co-morbidities (Department of Health, 2008). More recent studies have demonstrated similar findings, with between a quarter and half of prisoners likely to suffer from mental illness, with the proportion rising to between 70 and 90 per cent if substance misuse issues are included (private correspondence with the National Offender Management Service, 9 March 2016).

Many people with mental health conditions use drugs or alcohol to help them to deal with their symptoms.