Major Finding: A dose-response relationship was seen between younger onset age of cannabis use and greater risk of psychosis-related outcomes in sibling pairs.
Data Source: Prospective, sibling pair analysis of 3,081 adults born between 1981 and 1984.
Disclosures: None of the investigators had any financial conflicts of interest to report.
The use of cannabis at a younger age is associated with psychosis symptoms in early adulthood, according to an Australian study.
This is the first study to show the association in a subgroup of sibling pairs, “reducing the likelihood that the association was due to unmeasured shared genetic and/or environmental influences,” said Dr. John McGrath of the Queensland Brain Institute, Wacol, Australia, and his colleagues. The study also demonstrated a dose-response relationship between younger age at first use and higher risk of psychosis-related outcomes, they noted.
The researchers examined the link using data from a birth cohort of more than 7,000 mother-infant pairs who were first studied in 1981–1984 and followed 5, 14, and 21 years later.
The study comprised 3,801 of these infants and their close-in-age siblings. The latest follow-up occurred when they were aged 18–23 years. At that time, about 18% of the subjects said they had been using marijuana for 3 or fewer years, 16% said they had been using it for 4–5 years, and 14% said they had been using it for 6 or more years.
At final follow-up, 65 of these subjects had received diagnoses of nonaffective psychosis because they met the criteria for schizophrenia (53 subjects), persistent delusional disorder (3), or acute transient psychotic disorders (9). Another 233 subjects reported at least one visual or auditory hallucination.
Only subjects with the longest duration since first cannabis use (those who started using marijuana at age 15 years or younger) were at significantly increased risk for developing symptoms of nonaffective psychosis in young adulthood. Those who started using marijuana at that age were twice as likely to receive such a diagnosis than were subjects who said they had never used marijuana, the researchers said (Arch. Gen. Psychiatry 2010 March 1 [doi:10.1001/archgenpsychiatry.2010.6]).
Compared with subjects who did not use cannabis, those who used it at a younger age were 4 times more likely to score in the top quartile on the 21-item Peters et al. Delusional Inventory (PDI) and to report hallucinations on the CIDI.
Moreover, the longer the interval since first cannabis use, the higher the risk of these adverse psychosis-related outcomes. In a subsample of 218 sibling pairs, there was a significant association between earlier first use of cannabis and higher scores on the PDI. For every additional year since first exposure to marijuana, the sibling with the younger age at first use scored one item higher than the other sibling, the researchers said.
This study was not designed to determine causality.
Those who began marijuana use at age 15 or younger had double the risk of psychosis.
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