Frequent cannabis use could lead to lower IQ in teenagers

A study has found that adolescents who frequently consume cannabis may experience a decline in IQ over time. 

The study, led by researchers at RCSI University of Medicine and Health Sciences, revealed that there were declines of approximately 2 IQ points over time in those who consume cannabis frequently compared to those who didn’t.

Further analysis suggested that this decline in IQ points was primarily related to reduction in verbal IQ.

The research involved a systematic review and statistical analysis on seven longitudinal studies involving 808 young people who used cannabis at least weekly for a minimum of 6 months and 5,308 young people who did not use cannabis. 

In order to be included in the analysis, each participant had to have a baseline IQ score prior to starting cannabis use and another IQ score at follow-up. 

The young people were followed up until age 18 on average, although one study followed the young people until the age of 38.

“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 performance in school and college and later employment prospects,” commented senior author on the paper Professor Mary Cannon, Professor of Psychiatric Epidemiology and Youth Mental Health, RCSI.

Dr Emmet Power, Clinical Research Fellow at RCSI and first author on the study added: “Cannabis use during youth is of great concern as the developing brain may be particularly susceptible to harm during this period. The findings of this study help us to further understand this important public health issue.”

The study was carried out by researchers from the Department of Psychiatry, RCSI and Beaumont Hospital, Dublin (Prof Mary Cannon, Dr Emmet Power, Sophie Sabherwal, Dr Colm Healy, Dr Aisling O’Neill and Professor David Cotter).

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