Cannabis use by young people can be linked to decline in IQ

A senior author on the study said the use of cannabis at a young age can put young people at an increased risk of having serious mental illnesses later in life
Cannabis use by young people can be linked to decline in IQ

The results of the study have revealed that there was a declination of approximately 2 IQ points over time in those who use cannabis frequently. Photo: Getty

A new study has found that frequent cannabis use by young people can be linked to a decline in IQ.

The results of the study have revealed that there was a declination of approximately 2 IQ points over time in those who use cannabis frequently compared to those who didn’t use cannabis.

The paper, led by researchers at RCSI University of Medicine and Health Sciences, analysed 808 young people who used cannabis at least weekly for a minimum of six months and 5,308 young people who did not use cannabis.

Professor Mary Cannon, Professor of Psychiatric Epidemiology and Youth Mental Health, said the findings of the research provide further insight into the harmful neurological and cognitive effects of frequent cannabis use on young people.

“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.” 

The research involved a systematic review and statistical analysis on seven longitudinal studies.

In order to be included in the analysis, each study 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 on average until age 18, although one study followed the young people until age 38.

“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,” said Dr Emmet Power, Clinical Research Fellow at RCSI.

Further analysis has suggested that the decline of approximately 2 IQ was primarily related to a reduction in verbal IQ.

The study was carried out by researchers from the Department of Psychiatry, RCSI and Beaumont Hospital, and is published in Psychological Medicine.

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