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IQ decline found in adolescents with frequent cannabis use

By Nicole MacKee
Regular cannabis use in young people is associated with a two-point decline in intelligence quotient (IQ), Irish researchers have reported in Psychological Medicine.

In a systematic review and meta-analysis, researchers evaluated seven cohort studies with 808 young people who used cannabis at least weekly for six months and 5308 controls who did not use cannabis.

They found that frequent or dependent cannabis use in adolescence was associated with an average decline of about two IQ points.

While follow up was limited to age 18 years in six of the seven studies, one study followed participants to their mid-30s.

This study, the researchers noted, showed a dose–response relationship, indicating that a potential neurodevelopment impact of adolescent cannabis use may be underestimated in the systematic review.

Dr Janni Leung, Global Substance Use and Mental Health Research Fellow with the University of Queensland’s School of Psychology and the National Centre For Youth Substance Use Research, Brisbane, said adolescent cannabis use in Australia had been on the decline.

‘The latest Australian house-hold survey showed that 13% of 14- to 19-year-olds had used cannabis in the past 12 months in 2019,’ she said. ‘This is almost half of what it was in 2001, at that time it was 25%.’

Dr Leung said the latest study added to strong epidemiological evidence that frequent cannabis use, especially with younger age of initiation and long-term use, was related to impaired higher cognitive functions of memory, attention, organisation and integration of complex information.

‘There is some recovery after abstinence, but it is uncertain if cognitive function fully recovers or could be repaired with training after the cessation of cannabis use,’ she said.

Dr Leung said epidemiological studies of adolescents indicated that chronic heavy cannabis use may adversely affect psychosocial development, including having low academic achievements and dropping out of school.

‘Interpretation of this evidence is complicated because many of the risk factors for the adverse developmental outcomes may also precede cannabis use and make it more likely that a young person will engage in cannabis use,’ she said. ‘These include minor delinquency, other substance use, poor educational performance, nonconformity and poor social adjustment.’
Psychol Med 2021; 51: 194-200.