Substance use
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Drug and alcohol use

What is comorbid drug and alcohol use? 

Drug and alcohol misuse, abuse or dependence are concerns for people with schizophrenia due to the association with poor clinical and social outcomes, including high rates of suicide, HIV, homelessness, aggression and incarceration. Moreover, substance use places additional burden on patients, families, psychiatric services, and government resources due to high rates of treatment non-adherence and relapse. This topic covers outcomes for people with schizophrenia and comorbid substance use (termed ‘dual diagnosis’). Please also see the topic on rates of comorbid substance use, as well as substance use as a risk factor for schizophrenia.

What is the evidence on outcomes for people with schizophrenia and comorbid drug and alcohol use?

High quality evidence shows a small increase in positive symptoms, but a medium-sized reduction in negative symptoms in people with schizophrenia and any current substance use disorder compared to people with schizophrenia without a current substance use disorder. Moderate to high quality evidence finds patients with any current substance use are also more likely to have depressive symptoms.

Moderate to low quality evidence finds an increased risk of treatment non-adherence, relapse and re-hospitalisation in people with first-episode psychosis and cocaine, opiates, or ecstasy use. Patients with a mixed psychoactive substance use disorder or a cocaine use disorder also show increased extrapyramidal (movement) symptoms, particularly akathisia and tardive dyskinesia compared to patients without a substance use disorder.

For cannabis use, high quality evidence found a small to medium-sized decrease in negative symptoms in people with schizophrenia who recently abstained from cannabis use compared to people with schizophrenia with no cannabis use. There was a small increase in positive symptoms and hospital stay duration in people who continued cannabis use after the first onset of psychosis compared to non-users of cannabis. There were also higher rates of relapse in people who continued cannabis use compared to people who discontinued cannabis use after the first onset of psychosis. Cannabis use was also associated with and earlier age of onset of the disorder, more suspiciousness and unusual thought content in people at risk of psychosis.

For cognition, high quality evidence shows a small effect of lower current IQ, and a medium-sized effect of lower premorbid IQ in people with psychosis and current cannabis use compared to people with psychosis without current cannabis use. Moderate quality evidence also finds poorer verbal working memory in those currently using cannabis. For people with schizophrenia specifically, moderate to high quality evidence finds a medium-sized effect of better global cognition, processing speed, planning, and visual and working memory in those with any history of cannabis use, but not in those with current cannabis use. Similarly, high quality evidence shows a small to medium-sized increase in global cognition, processing speed, planning, visual and working memory, attention, and psychomotor skills in people with psychosis and a polysubstance or cannabis use disorder compared to people with psychosis with no substance use disorder. For people with psychosis and an alcohol use disorder, moderate quality evidence finds more impaired working memory compared to people with psychosis and no substance use disorder.

March 2022

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Last updated at: 6:12 pm, 28th March 2022
To view documentation related to this topic download the files below
Fact Sheet Technical Commentary
Tags:  Dual diagnosis

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