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Cannabis harm to teenagers' brains 'overstated', finds study

Marijuana’s impact on adolescents’ brain development and mental health remains a major concern

Alex Matthews-King
Health Correspondent
Wednesday 18 April 2018 18:36 BST
Cannabis fans in London celebrate 420 at Hyde Park gathering

Fears that cannabis causes irreparable harm to teenager’s brains have been stoked by trials which “overstated” the effects on intelligence and other functions, according to a review which found little ill-effect after three days abstinence.

Studies have shown it is 114 times less harmful than alcohol, but marijuana’s impact on adolescents’ brain development and mental health is a major concern for policy makers in debates over legalisation.

This is a key time developmental period and studies have found negative impacts on attention, learning, memory and organisation in heavy or frequent cannabis users.

But it now appears that these studies may have just been identifying impairment caused by residual effects of the drug or withdrawal symptoms.

A meta-analysis by University of Pennsylvania researchers looked at 69 studies, which all tested the impact of marijuana smoking in adolescents and young adults, and found little to no long-term harm.

Although it did not look at mental health impacts, such as psychosis, the authors of the study published in the journal JAMA Psychology today, found the “persistence and magnitude of impact” on teenagers had been overblown.

For the test the authors looked at results of cognitive testing on learning, memory, speed of information processing, language and motor skills for 2,152 cannabis users and 6,575 who had minimal use.

It found there was a “small but statistically significant” effect associated with heavy or frequent use.

But this effect shrank in studies that required their cannabis-smoking participants to abstain from smoking before testing, and a period of more than 72-hours abstinence meant there was no significant impact on their performance.

It also found no evidence that earlier use, particularly in teenagers, had a more serious long-term effect.

“In light of the changing perceptions of cannabis use and an evolving policy landscape surrounding cannabis, understanding the potential risks of cannabis use for mental health and brain functioning is of paramount importance,” the authors conclude.

“[Our] results indicate that previous studies of cannabis in youth may have overstated the magnitude and persistence of cognitive deficits associated with use.

“Reported deficits may reflect residual effects from acute use or withdrawal.”

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