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26-05-2010 | Mental health | Article

Cannabis use increases psychosis-related outcomes


Free abstract

MedWire News: Researchers report evidence from a sibling pair analysis that supports the association between early cannabis use and psychosis-related outcomes.

“Sibling pair designs capitalize on between-sibling differences while reducing the influence of unmeasured confounding factors, since differences are less likely to be attributable to shared genetic and environmental exposures,” explain John McGrath, from the Queensland Centre for Mental Health Research in Australia, and colleagues.

The researchers examined three psychosis-related outcomes (nonaffective psychosis, hallucinations, and Peters et al Delusions Inventory [PDI] score) in 3801 individuals born between 1981 and 1984 who completed the 21-year follow-up of the Mater-University Study of Pregnancy.

In all, 65 individuals were diagnosed with nonaffective psychosis, while 233 endorsed at least one hallucination item on the Composite International Diagnostic Interview. The average total PDI score was 5.1.

Of the participants, 17.7% reported cannabis use for 3 or fewer years, 16.2% for 4 to 5 years, and 14.3% for 6 or more years.

After accounting for gender, age, parental mental illness, and the presence of hallucinations at the 14-year follow-up, individuals who had used cannabis for 6 or more years had an odds ratio of 2.2 for nonaffective psychosis, 4.3 for being in the highest quartile of PDI score (endorsed eight or more items), and 2.8 for hallucinations, compared with individuals who had never used cannabis.

There were significant linear trends between cannabis use and all three psychosis-related outcomes, with longer duration of use associated with greater risk.

This relationship was confirmed in a sub-analysis of 10 sibling pairs discordant for nonaffective psychosis, with the sibling with more years since first cannabis use more likely to have higher PDI total scores.

Indeed, for every additional year since first exposure to cannabis, the sibling with the earlier age at first use scored approximately one PDI item higher than their sibling.

This significant relationship between years of cannabis use and PDI scores persisted in 100 sibling pairs who both used cannabis.

The researchers note in the Archives of General Psychiatry that there was also evidence of reverse causality, with individuals reporting hallucinations at the 14-year follow-up twice as likely to be using cannabis on a daily basis at the 21-year follow-up as those without hallucinations. In addition, analyses adjusting for depressive- and anxiety-related disorders suggested that these disorders may mediate or moderate the association between cannabis use and psychosis-related outcomes.

This highlights the “complexity of the relationship,” they write.

The team concludes: “This study provides further support for the hypothesis that early cannabis use is a risk-modifying factor for psychosis-related outcomes in young adults.”

MedWire ( is an independent clinical news service provided by Current Medicine Group, a trading division of Springer Healthcare Limited. © Springer Healthcare Ltd; 2010

By Lucy Piper

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