Frequent cannabis use may decline IQ among young people

The findings, published in the journal Psychological Medicine, revealed that there were declines of approximately 2 IQ points over time in those who use cannabis frequently compared to those who didn't use cannabis.

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

"Cannabis use during youth is of great concern as the developing brain may be particularly susceptible to harm during this period," said researcher Emmet Power from the Royal College of Surgeons in Ireland (RCSI).

"The findings of this study help us to further understand this important public health issue," Power added.

For the study, the team involved 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 5308 young people who did not use cannabis.

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 until age 18 on average although one study followed the young people until age 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," said researcher Mary Cannon, Professor at RCSI.

"Loss of IQ points early in life could have significant effects on performance in school and college and later employment prospects," Cannon added.