Researchers from the Royal College of Surgeons in Ireland have found that people who use cannabis frequently during adolescence may experience a higher decline in IQ over time than those who don't use the substance.
"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," says Mary Cannon, senior author of the paper.
For the study, the researchers performed a systematic and statistical analysis of seven longitudinal studies. Altogether, they analyzed data from 808 young people who used cannabis on a weekly basis for a minimum of six months. To be included in the study, each person had to have a baseline IQ score prior to starting to use cannabis, as well as another at a follow-up.
The researchers also analyzed data from 5,308 people who did not use the substance. All participants were monitored throughout adolescence until an average age of 18 apart from one study, in which participants were followed until age 38.
All in all, the researchers found that people who used cannabis frequently during adolescence tended to have an average decline of around 2 IQ points later on. The researchers found these losses to be primarily in verbal IQ.
"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," says Dr Emmet Power, first author of the study.
The researchers now say that further studies should be conducted that assess participants over longer periods of time to understand the longer-term impacts of cannabis on IQ.