More About Cannabis and Psychosis

19 de julho de 20093min5

Acute exposure increases psychosis-like symptoms, especially among those with high-trait schizotypy.

Numerous studies have associated regular cannabis use with psychosis, but the causal relation is unclear. To explore this further, two research groups recently examined the effects of acute cannabis exposure.

On two separate occasions, Mason and colleagues gave intravenous 9-tetrahydrocannabinol (THC) or placebo to 22 healthy men who had no histories of psychiatric disorders or substance dependence. The men then answered mood and psychotic-symptom questionnaires and underwent cognitive testing. Compared with placebo, THC was associated with significant increases in psychotic symptoms (mainly ideas of reference), dysphoria, tiredness, and anxiety, and with impaired immediate recall and executive functioning. About 50% of participants had increased psychotic symptoms with THC. Psychotic symptoms were not associated with anxiety or cognitive impairment.

Morrison and colleagues assessed self-reported schizotypal symptoms in 140 recreational cannabis users while they were acutely intoxicated and when abstinent for at least 24 hours; trait schizotypy was also measured during abstinence. Controls (144 nonusers of psychotropic drugs) completed the same questionnaires, without cannabis exposure. While intoxicated, cannabis users endorsed more schizotypal symptoms than controls, but their responses were similar to those of controls when abstinent. Cannabis users in the upper quartile for trait schizotypy had significantly higher symptom scores with acute cannabis use than did those in the lowest quartile and showed the greatest change in symptoms between test days. Excluding participants who used other drugs did not alter the results.

Comment: Previous studies have also showed that acute cannabis use can induce psychosis-like symptoms, even in healthy volunteers without preexisting Axis I psychotic disorders. This effect seemed more pronounced in individuals who scored high on a trait schizotypy questionnaire in the Morrison study and was not related to other effects of cannabis, such as anxiety or cognitive impairment, in the Mason study. Different forms of cannabis have different THC and cannabidiol concentrations and might vary in risk for psychotic symptoms. Still, these studies add evidence on the potentially harmful effects of cannabis. Clinicians should ask about cannabis use in patients exhibiting psychotic symptoms and should address cannabis use as part of treatment.

— Deborah Cowley, MD

Published in Journal Watch Psychiatry July 13, 2009


Mason O et al. Acute cannabis use causes increased psychotomimetic experiences in individuals prone to psychosis. Psychol Med 2009 Jun; 39:951.

Medline abstract (Free)

Morrison PD et al. The acute effects of synthetic intravenous 9-tetrahydrocannabinol on psychosis, mood and cognitive functioning. Psychol Med 2009 Apr 1; [e-pub ahead of print]. (


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