Teenager leaning against wall thinking

Link between cannabis use and IQ decline

  • Research
  • Society

Frequent cannabis use in adolescence might result in a gradual decline in IQ levels, according to a research study from RCSI University of Medicine and Health Sciences.

The study 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. It revealed a decline of approximately two IQ points over time in those who did use cannabis compared to those who did not.

Researchers recorded a baseline IQ score for each participant prior to starting cannabis use and another IQ score at follow-up. The young people were monitored until age 18 on average, although one study followed the young people until age 38.

It is already understood that young people who use cannabis frequently tend to have worse outcomes in life than their peers and are at an increased risk for serious mental illnesses such as schizophrenia. This research provides greater insight into the cognitive effects that cannabis has on young people and the negative impact its frequent use could have on a young person’s future education and employment prospects.

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

Published in Psychological Medicine, the research was led by Professor Mary Cannon, Professor of Psychiatric Epidemiology and Youth Mental Health at RCSI, Professor David Cotter, Professor of Psychiatry at RCSI, and Dr Emmet Power, Clinical Research Fellow at RCSI.

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