Teenage warning: heavy cannabis use ‘lowers your IQ’

The study of 1,037 New Zealanders, who were monitored over a 40-year period, found that teenagers who became heavy cannabis users suffered an average eight-point decline in IQ between adolescence and adulthood.

These were people who smoked cannabis at least four times a week for more than 20 years.

They had significantly more problems with attention span and memory than non-users, and cutting down or giving up later did not
fully reverse these effects.

Those who starting smoking cannabis as adults did not suffer the same outcomes, which suggests that young brains are more susceptible to drug-related damage than older ones.

It’s such a special study that I’m fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains. Professor Terrie Moffitt, Institute of Psychiatry

The international study has been published in the Proceedings of the National Academy of Sciences journal.

Confidentiality

Professor Terrie Moffitt, from the Institute of Psychiatry at King’s College London, who took part in the study, said: “Participants were frank about their substance abuse habits because they trust our confidentiality guarantee, and 96 per cent of the original participants stuck with the study from 1972 to today.

“It’s such a special study that I’m fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains.”

Madeleine Meier, who also worked on the study, said the message was clear: “Marijuana is not harmless, particularly for adolescents.”

Researchers followed the progress of participants from birth to the age of 38, carrying out neuropsychological tests at age 13 and again at the end of the study. Cannabis use was recorded at five intervals from the age of 18 onwards.

IQ decline

They wrote: “The most persistent adolescent-onset cannabis users evidenced an average eight-point IQ decline from childhood to adulthood.

“Prevention and policy efforts should focus on delivering to the public the message that cannabis use during adolescence can have harmful effects on neuropsychological functioning, delaying the onset of cannabis use at least until adulthood, and encouraging cessation of cannabis use particularly for those who began using cannabis in adolescence.”

Professor Robin Murray, from the Institute of Psychiatry, said: “We have known for some time that heavy use of cannabis increases risk of schizophrenia-like psychoses, but this remains a relatively rare outcome so it’s not so important from a public health point of view.

“There are far fewer studies on its effect on minor psychiatric illness or on everyday life.

Less successful

“However, there are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations.

People with high IQs tend to be better educated, earn more and live longer lives than others.

Madeleine Meier said: “Somebody who loses eight IQ points as an adolescent may be disadvantaged … for years to come.”