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Comorbidity of mental disorders and substance use: a brief guide for the primary care clinician

7.3 Major clinical issues with psychosis and cannabis/ hallucinogen use

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• Cannabis can induce or cause a temporary psychotic state that clears within several days in individuals with no prior diagnosis of psychosis.
• Cannabis can trigger psychosis in individuals who are at risk of psychosis.
• Cannabis can worsen psychotic symptoms in those individuals who have a current diagnosis of psychosis.
• People with psychotic disorders should avoid cannabis and be counselled against its use. Brief interventions should be delivered for people with psychosis who may be using even small amounts of cannabis.
• In an acute psychotic episode caused by cannabis use, cessation of use will result in resolution of the episode.
• Psychoeducation and CBT orientated programs have shown promise in reducing cannabis use in first-episode psychosis patients.
• In the absence of other proven forms of treatment, CBT is, at present, the most widely employed form of treatment for cannabis use.

7.3.1 Effects of cannabis and other hallucinogens on psychiatric disorders

• One of the most commonly used substances in individuals with psychosis is cannabis, with individuals with schizophrenia and bipolar disorder quite often receiving an additional diagnosis of cannabis dependence^{(1, 2, 264, 265, 268, 279, 280)}.
• Duration of cannabis use in people with bipolar disorder is associated with the duration of mania⁽²⁸¹⁾.
• Cannabis increases the risk of tardive dyskinesia⁽²⁾.
• There is growing evidence that cannabis use is a significant contributory factor in psychosis^{(126, 282-285)}:
  • Cannabis can induce or cause a temporary psychotic state that clears within several days in individuals with no prior diagnosis of psychosis^{(2, 14, 15)}.
  • Cannabis can trigger psychosis in individuals who are at risk of psychosis^{(286, 287)}.
  • Cannabis can worsen psychotic symptoms in those individuals who have a current diagnosis of psychosis^{(15, 286-291)}.
  • Cannabis use is associated with an earlier onset of psychosis^{(264, 265, 279)}.
  • There is an association between cannabis and psychosis. However, when rates of cannabis use were increasing in Australia, no increase in the rates of schizophrenia was observed⁽²⁸⁶⁾.
• People with psychosis generally do not use cannabis in a self-medicating manner to reduce psychotic symptoms. Reported reasons for use include social isolation, lack of emotion or feeling for others, lack of energy, difficulty sleeping, depression, anxiety, agitation, tremor or shaking and boredom. These symptoms may occur as part of the psychotic illness or may be due to additional anxiety or depressive illnesses or side effects of medication^{(15, 292, 293)}.

7.3.2 Interactions between cannabis and other hallucinogens and therapeutic agents for psychotic disorders

• It is unclear whether chronic cannabis consumption induces the metabolism of the antipsychotics and reduces plasma concentrations in a similar manner to tobacco⁽²⁹⁴⁾.
• Cannabis can exacerbate the sedative effects of antipsychotics and mood stabilisers such as carbamazepine, lithium and sodium valproate^x.
• Cannabis will exacerbate the sedative effects of tricyclic antidepressants and benzodiazepines used to treat breakthrough depression⁽²⁾ and anxiety in psychosis which increases the risk of overdose^x.

7.3.3 Management approaches to comorbid psychotic disorders and cannabis use

Prevention

• In general, people with psychotic disorders should avoid cannabis and be counselled against its use. Brief interventions should be delivered for people with psychosis who may be using even small amounts of cannabis.

Initial assessment

• Despite the common diagnosis of comorbid psychosis and cannabis dependence, there has been little research to define specific unique management approaches. It is generally thought that the best outcomes are achieved when treatment for both conditions is integrated.
• An attempt should be made to distinguish between people with:
  • An acute psychotic episode caused by cannabis use.
  • A first episode of a psychotic disorder.
  • An acute episode which has been precipitated by cannabis use in someone with an established chronic psychotic disorder.
• In an acute psychotic episode caused by cannabis use, cessation of use will result in resolution of the episode. The short-term use of an antipsychotic medication or benzodiazepines may be indicated, depending on the level of distress. The duration of use should be titrated against the symptoms.

Treatment

• Psychoeducation and CBT orientated programs have shown promise in reducing cannabis use in patients experiencing their first episode of psychosis***⁽²⁹⁵⁾.
• Abstinence from cannabis is a difficult goal to achieve in cannabis dependent people⁽¹²⁸⁾. This is particularly the case with people with psychosis due to the social isolation they often experience. This social isolation has been reported as a major motivator for continued use^{(15, 292, 293)}.
• In the absence of other proven forms of treatment, CBT is, at present, the most widely employed form of treatment for cannabis use****⁽¹²⁸⁾.
• Preliminary studies have shown clozapine to be more effective than risperidone in reducing cannabis use in people with psychotic disorders*⁽²⁹⁶⁾.
• Early preliminary data in one trial also shows some benefit for olanzapine in cannabis induced psychotic disorder***⁽²⁹⁷⁾.
• Benzodiazepine use for acute symptom control should be minimised as those with substance use disorders are at a greater risk of abusing benzodiazepines⁽¹⁹⁶⁾.

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