Alcohol & other drugs services in Victoria
Treatment & support

Cannabis & psychosis - information for health care workers

What is the evidence that cannabis use is associated with psychosis?

It should be noted from the outset that:

• much of the evidence for cannabis-associated psychosis has been based on subjects who have used large doses of cannabis and/or used cannabis chronically
• a great deal of the data has been derived from small studies, some of which have contained inherent flaws
  and
  
• the findings of some of the larger studies conducted over the last 20 years have varied considerably

That established, the following findings are noteworthy:

• Some studies have investigated how often psychosis is diagnosed in cannabis users and non-users. One such study revealed that people who used cannabis were more likely to have a psychotic diagnosis than people who did not use cannabis.
  
  Surveys have also indicated that people who admit to using cannabis report more psychotic symptoms. One study found that in the general population, using cannabis doubled the likelihood of reporting one or more psychotic experiences in a 12-month period.

The risk of developing psychosis is believed to be increased by:

• the use of stronger forms of cannabis(for example, hashish, skunk)
• prolonged cannabis use
  the use of greater amounts of cannabis
  
• a personal history of psychotic illness
  and/or
  
• a family history of psychotic illness.

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What type of psychoses are associated with cannabis use?

It is believed that cannabis use may cause a condition known as a drug-induced psychosis or cannabis psychosis which:

• can last up to a few days
• is often characterised by hallucinations, delusions, memory loss and confusion
• usually results from prolonged or heavy cannabis use
  and
  
• responds well to treatment.

However, in some cases, cannabis use may contribute to the development of a psychosis such as schizophrenia which:

• constitutes a serious mental illness for the majority of people with the disorder
• is characterised by hallucinations, delusions, social withdrawal, paranoia, self-neglect, thought withdrawal, and speech disorders like associative or incoherent speech
  and
  
• tends to respond less well to treatment than cannabis psychosis.

Most of the research in this area has investigated cannabis in people with schizophrenia.

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Does cannabis use cause psychosis ?

• there are a number of risk factors which are believed to contribute to the development of a psychosis, one of which is believed to be cannabis use
• the peak age range during which people are more vulnerable to developing a psychosis (16-30 years of age) tends to be the same age range during which people tend to use cannabis
• a very large study of 50,465 Swedish army conscripts determined their cannabis use at age 18 and followed those individuals for the next 15 years. Recruits who had tried cannabis by age 18 had 2.4 times the risk of being diagnosed with schizophrenia in the following 15 years than those who had never used cannabis. The risk increased if the conscript used greater quantities of cannabis

However, of the conscripts who developed schizophrenia:

• most had never used cannabis
  and
  
• only 7.7 per cent were heavy cannabis users
  

Therefore, cannabis use is only one possible factor contributing to an increased risk of developing schizophrenia or other psychosis. This is especially true when one considers that most people who develop a psychosis have never used cannabis

In addition, cannabis use at age 18 might be a consequence of emerging psychosis (that is, psychosis precedes cannabis use, not vice versa).

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How does cannabis use affect psychosis?

• Cannabis use can prolong the duration of the symptoms of mental illness and can lower a person's chances of recovering from a psychotic episode
• The continuing use of cannabis after the development of a psychosis is believed to deleteriously affect the course of the psychosis
• The use of other drugs in addition to cannabis (such as amphetamines and alcohol) may produce complex, interactive effects on the development and the course of a psychosis
• It is believed that cannabis use may increase the risk of a relapse.

Do symptoms improve if a person with a psychosis stops using cannabis?

• Anecdotal indications would suggest that symptoms do improve if a person with a psychosis stops using cannabis, however, scientific evidence is weak. Only one study has been well-designed. This research found that reducing the misuse of all drugs (including alcohol and other illicit drugs) improved social functioning and psychotic symptoms.

Who is most at risk from cannabis use?

• People most at risk are those with a family history of psychotic illness or those who have already experienced a psychotic episode
• Therefore people with a family or a personal history of psychotic illness should avoid drugs like cannabis completely or at least attempt to minimise their use.

What about the effect of other drugs?

• Although this information focuses on cannabis, the effect of other legal and illicit drugs plays an important role, particularly for people with a psychosis who are poly-drug users. For example, some evidence suggests that substances such as alcohol and amphetamines have a greater effect than cannabis in the development of a psychosis.

Where can my clients turn for help?

If a person or client presents with psychotic symptoms, medical attention should be sought or provided immediately.

For confidential information and referral, friends and families can ring 1300 85 85 84, Monday-Friday 9AM-5PM, free call.

In addition, a single free copy of our Cannabis Diary can be obtained by contacting DrugInfo on 1300 85 85 84.

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Is any research currently being undertaken to investigate the link between cannabis use & psychosis?

The Victorian Government has funded research involving clinical and neurobiological studies, in an attempt to establish whether there is a link between cannabis use and psychosis.

Further reading

Andreasson, S. et al. 'Cannabis and schizophrenia: A longitudinal study of Swedish conscripts' Lancet, 2, 1483-1486, (1987).

Beaubruhn, M. and Knight, F. 'Psychiatric assessment of 30 chronic users of cannabis and 30 matched controls' American Journal of Psychiatry 130, 309-311, (1973).

Hambrecht, M. and Hafner, H. 'Substance use and the onset of schizophrenia' Biological Psychiatry 40, 1155-1163, (1996).

Hall, W. Cannabis Use and Psychosis National Drug and Alcohol Research Council Sydney Australia (1997).

Imade, A.G.T. and Ebie, J.C. 'A retrospective study of symptom patterns of cannabis-induced psychosis' Acta Pyschiatrica Scandinavia 83, 134-136, (1991).

Jerrell, J. and Ridgely, M.S. 'Comparative effectiveness of three approaches to serving people with severe mental illness and substance abuse disorders' Journal of Nervous and Mental Disease ; 183, 566-576, (1995).

Linszen, D.H., et al. 'Cannabis use and the course of recent-onset schizophrenic disorders' Archives of General Psychiatry 51, 273-279, (1994).

Mathers, D.C. et al. 'Cannabis use in a large sample of acute psychiatric admissions' British Journal of Addiction 86: 779-784, (1991).

McGuire P. et al. 'Cannabis and acute psychosis' Schizophrenia Research , 13: 161-168, (1994).

McGuire P. et al. 'Morbid risk of schizophrenia for relatives of patients with cannabis associated psychosis' Schizophrenia Research , 15, 277-281, (1995).

Mueser, K.T. 'Comorbidity and substance abuse: Implications for treatment' Journal of Consulting and Clinical Psychology 60, 845-856, (1992).

Rolfe, M. et al.’ Psychosis and cannabis abuse in The Gambia: A case control study' British Journal of Psychiatry 163, 789-801, (1993).

Rottanburg, D. et al. 'Cannabis-associated psychosis with hypomanic features' Lancet , 2, 1364-1366, (1982).

Thacore, V.R. and Shukla, S.R.P. 'Cannabis Psychosis and Paranoid Schizophrenia' Archives of General Psychiatry 33, 383-386, (1976).

Thornicroft, G. 'Cannabis and psychosis: Is there epidemiological evidence for association?' British Journal of Psychiatry 157, 25-33, (1990).

Tien, A.Y. and Anthony, J.C. 'Epidemiological analysis of alcohol and drug use as risk factors for psychotic experiences' Journal of Nervous and Mental Disease 178: 473-480, (1990).

Acknowledgements

The booklet (which was basis for this page) has been based on the report entitled, Cannabis Use and Psychosis (1997) by Professor Wayne Hall of the National Drug and Alcohol Research Centre, Sydney, Australia.

The NDARC report was initially modified into booklet form by the Drug Wise consultancy and subsequently developed and refined by the Expert Reference Group on Cannabis and Psychosis.