Cannabis use by teenagers erodes IQ over time, research shows

Frequent users lose about two points, mainly effecting verbal dexterity, Dublin team finds

Teenagers who regularly use cannabis may suffer a decline in their IQs over time, new research has established.

The study, analysing neurological and cognitive effects of frequent cannabis use on young people, was carried out by researchers from the department of psychiatry at the Royal College of Surgeons Ireland and Beaumont Hospital, Dublin.

The results showed declines of approximately two IQ points over time in those who use cannabis frequently compared to those who did not. Further analysis suggested this decline to be primarily related to a reduction in verbal IQ.

Longitudinal studies

The research involved a systematic review and statistical analysis of seven longitudinal studies involving 808 young people who used cannabis at least weekly for a minimum of six months and 5,308 young people who did not.


Participants were then monitored until the age of 18 on average although one study followed young people until age 38.

Mental illnesses

"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," said Mary Cannon, the senior author and professor of psychiatric epidemiology and youth mental health at the RCSI.

The research, published in the Psychological Medicine journal, was funded by a YouLead collaborative doctoral award from the Health Research Board (Ireland) and a European Research Council Consolidator Award.

Mark Hilliard

Mark Hilliard

Mark Hilliard is a reporter with The Irish Times