Low IQ risk revealed for young users of cannabis


TEENAGE YEARS are the worst time in a person’s life to smoke cannabis, a prominent Irish child and adolescent psychiatrist has said.

Dr Bobby Smyth was reacting to a study concerning the drug’s effect on the adolescent brain. He works for the HSE at the Youth Drug and Alcohol Service in Tallaght and referred to the growing evidence that cannabis has a long-term negative impact on the developing brain.

A study published yesterday in American journal Proceedings of the National Academy of Sciences found cannabis can lower the IQ of young teenagers and may cause permanent mental impairment.

The most persistent users suffered an average eight-point decline in IQ between adolescence and adulthood.

Users experienced significantly more attention and memory problems than non-users, the study found. This was the case even after taking account of different educational backgrounds and use of alcohol and other drugs.

The tests showed wide-ranging mental decline among men and women who began taking cannabis at a young age and continued using the drug regularly for more than 20 years.

The participants were members of the Dunedin Study, a longitudinal survey of more than 1,000 people born in New Zealand in 1972 and 1973.

Neuropsychological testing was conducted on participants at age 13 before they began using cannabis and again at 38. Their cannabis use was recorded at regular intervals up to the age of 38.

The study, led by US psychologist Dr Madeline Meier, from Duke University in Durham, North Carolina, found persistent use was associated with neuropsychological decline, even after controlling it during years of education.

Persistent cannabis use appeared to affect everyday mental functioning. Friends and relatives questioned by researchers said users experienced significant attention and memory problems.

“Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline,” the study noted. Moreover, while quitting the drug prevented further impairment, it did not appear to restore normal mental functioning for those who began smoking cannabis in adolescence.

Cognitive abilities of those who started using the drug in their 20s also suffered while they were smoking. But if they gave up at least a year before an IQ test at age 38 their intelligence recovered.

Recent studies have shown the use of cannabis among under 18s in Ireland has declined. The Health Behaviour in School-aged Children study, carried out by the health promotion research centre at NUI Galway and published earlier this year, found 8 per cent of those who took part had used cannabis in the past 12 months, down from 16 per cent in 2006.

Dr Smyth said adolescents he sees suffer from the drug’s short-term effects, these include a lack of motivation. But this fresh evidence revealed cannabis was also affecting cognition a decade or two later. The study’s results were unsurprising, he said, and were part of a growing body of evidence on how the drug affects the brain.

He said recent studies, including from the Royal College of Surgeons in Ireland, demonstrated the connection between cannabis use in adolescence and schizophrenia.

“It doubles your risk of developing schizophrenia as an adult,” he said. And though the risk was still low, rising from 1 per cent to 2 per cent, the condition was a “devastating mental health problem” and anything that could be done to avoid it should be done, he said. “The message is if you insist on smoking cannabis, leave it until your mid-20s,” he added.