The term psychosis is used to describe a group of severe mental health disorders characterised by the presence of delusions and hallucinations that disrupt a person's perception, thoughts, emotions and behaviour. The main forms of psychosis are schizophrenia (including schizoaffective disorder, schizophreniform disorder and delusional disorder), bipolar disorder or other affective psychosis.
Substance misuse is a broad term encompassing, in this guideline, the harmful use of any psychotropic substance, including alcohol and either legal or illicit drugs. Such use is usually, but not always, regarded as a problem if there is evidence of dependence, characterised by psychological reinforcement of repeated substance-taking behaviour and, in some cases, a withdrawal syndrome. However, substance misuse can be harmful without dependence, especially among people with a coexisting psychosis.
Approximately 40% of people with psychosis misuse substances at some point in their lifetime, at least double the rate seen in the general population. In addition, people with coexisting substance misuse have a higher risk of relapse and hospitalisation, and have higher levels of unmet needs compared with other inpatients with psychosis who do not misuse substances.
Substance misuse among individuals with psychiatric disorders is associated with significantly poorer outcomes than for individuals with a single disorder. These outcomes include worsening psychiatric symptoms, poorer physical health, increased use of institutional services, poor medication adherence, homelessness, increased risk of HIV infection, greater dropout from services and higher overall treatment costs. Social outcomes are also significantly worse, including greater homelessness and rooflessness, a higher impact on families and carers, and increased contact with the criminal justice system.
People with psychosis commonly take various non-prescribed substances as a way of coping with their symptoms, and in a third of people with psychosis, this amounts to harmful or dependent use. The outcome for people with psychosis and coexisting substance misuse is worse than for people without coexisting substance misuse, partly because the substances used may exacerbate the psychosis and partly because substances often interfere with pharmacological or psychological treatment. This guideline aims to help healthcare professionals guide people with psychosis and coexisting substance misuse to stabilise, reduce or stop their substance misuse, to improve treatment adherence and outcomes, and to enhance their lives.
As well as primary and secondary services, this guideline also applies to services that are delivered by the third sector and commissioned by the NHS.
 Rooflessness here refers to living rough or on the streets, whereas homelessness encompasses people who are living in shelters.