Cannabis Use in Early Adolescence Especially Neurotoxic

Caroline Cassels

June 16, 2011

June 16, 2011 — Chronic cannabis use that starts before age 15 may have a more deleterious effect on cognitive functioning than chronic use that starts later, new research shows.

A study conducted by Brazilian investigators from the Federal University of Sao Paulo showed that early-onset (before age 15 years) chronic cannabis users had significantly poorer executive functioning than late-onset (after age 15 years) users.

"Our findings indicate that early-onset chronic cannabis users are cognitively impaired compared with controls, suggesting that early exposure to cannabis is associated with more adverse effects on the brain. These results are in line with previous studies examining cognitive effects associated with early cannabis exposure," the authors, led by Maria Alice Fontes, PhD, write.

The study is published in the June issue of the British Journal of Psychiatry.

Although many studies have suggested that adolescence is a period of particular vulnerability to neurocognitive effects associated with substance misuse, the investigators note that few large studies have measured differences in cognitive performance between chronic users who started before age 15 and their counterparts who started after age 15.

"We hypothesized that the early-onset group would perform poorly on cognitive tests evaluating executive functioning compared with both the late-onset group and the healthy controls," the authors write.

The researchers recruited 104 chronic cannabis users — 49 early-onset users and 55 late-onset users — who were seeking treatment at a substance use disorder program and a group of 44 healthy controls.

All participants were interviewed to provide a complete medical and psychiatric history and underwent a battery of neuropsychological tests that included the Stroop Test, Wisconsin Card Sorting Test, Frontal Assessment Battery, and Wechsler Adult Intelligence Scale–Revised.

The median age of the early-onset and late-onset groups was 30 years. For the control group, the median age was 27 years. The estimated average lifetime consumption was 6790 joints for the early-onset group and 5160 joints for the late-onset group.

The researchers report that there were no differences in IQ, vocabulary, or block design results when comparing the early-onset, late-onset, and control groups. However, they note, "the early-onset, but not the late-onset, group performed poorly on selected executive tasks compared with controls. In addition, the early-onset group performed poorly on the Frontal Assessment Battery compared with both the late-onset and the control groups."

The study also showed that the early-onset group had poor performance on the Stroop Test and perseverative responding as measured by the Wisconsin Card Sorting Test. These findings suggest that "exposure to cannabis in early adolescence may lead to lower flexibility."

"This could explain why chronic users find it difficult to maintain treatment and to discontinue substance use despite evident physical and psychological consequences," the authors write.

The authors note that it is unclear whether starting cannabis use at a later age affects cognition.

"It is possible that participants who start to use cannabis at a later age may use different neural networks and compensate for their deficits more than individuals who initiated the use of cannabis at an earlier stage of brain development."

They further suggest that with prolonged abstinence it may be possible to reverse the effects of chronic marijuana use on the altered brain network.

The authors have disclosed no relevant financial relationships.

Br J Psychiatry. 2011;198:442-447. Abstract

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