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Early Marijuana Use Increases Psychosis Risk in Young Adults

Study looks at cannabis-psychosis association in sibling pairs to minimize confounding variables

MONDAY, March 1 (HealthDay News) -- The early use of marijuana increases the risks for schizophrenia, delusions and hallucinations in young adulthood, according to a study published online March 1 in the Archives of General Psychiatry.

John McGrath, M.D., of the University of Queensland in St. Lucia, Australia, and colleagues studied 3,801 subjects born between 1981 and 1984 from the Mater-University Study of Pregnancy, including a nested cohort of 228 sibling pairs. The researchers assessed self-reported cannabis use at age 21 years and analyzed the association of duration since first cannabis use to three psychosis-related outcomes: non-affective psychosis (such as schizophrenia and delusional disorder), hallucinations, and the Peters et al Delusions Inventory score.

The researchers found that, for subjects with a duration since first cannabis use of six or more years, there was an increased risk of the three psychosis-related outcomes: adjusted odds ratio for non-affective psychoses, 2.2; being in the highest quartile of Peters et al Delusions Inventory score, 4.2; and hallucinations, 2.8.

"Early cannabis use is associated with psychosis-related outcomes in young adults. The use of sibling pairs reduces the likelihood that unmeasured confounding explains these findings. This study provides further support for the hypothesis that early cannabis use is a risk-modifying factor for psychosis-related outcomes in young adults," the authors write.

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