Teenage cannabis smoking ‘causes a drop in IQ’

SMOKING cannabis frequently during adolescence may lead to a drop in IQ, scientists say.

Those who use the drug often in their teenage years have an IQ that is two points lower on average when they are 18 than their peers who abstain, researchers at the RCSI University of Medicine and Health Sciences in Dublin found.

Smoking cannabis appeared to impact verbal IQ – communication abilities – but not ‘performance’ IQ, the aptitude to work out puzzles, codes and other non-verbal problems.

While the scientists described the overall decline as modest, they said it was ‘similar in size to effect sizes seen in exposure to lead, an environmental toxin, in childhood’.

Mary Cannon, professor of psychiatric epidemiology and youth mental health at RCSI, said: ‘ Previous research tells us that young people who use cannabis frequently have worse outcomes in life than their peers and are at increased risk for serious mental illnesses like schizophrenia.

‘ Loss of IQ points early in life could have significant effects on

‘It could have significant effects on job prospects’

performance in school and college and later employment prospects.’

Dr Emmet Power, who also worked on the paper, published in the journal Psychological Medicine, added: ‘Cannabis use during youth is of great concern as the developing brain may be particularly susceptible to harm during this period.’

The research involved a comprehensive review of previous work on the subject, particularly seven studies that followed 808 young people who used cannabis at least once a week for a minimum of six months and 5,308 teens who did not use the drug.

All participants had an IQ test at the start of the period they were followed and at the end.

The researchers stressed the drop in IQ among cannabis users was ‘not likely to be attributed to alcohol use’.