A systematic review and meta-analysis of seven longitudinal studies published in the journal Psychological Medicine shows evidence for an almost two-point decline in IQ associated with frequent or dependent cannabis use in adolescence.
Power et al. revealed that there were declines of approximately two IQ points over time in young people 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. Image credit: Felipe B. Marques.
Cannabis is the most frequently used illicit substance worldwide, with the prevalence of lifetime cannabis use highest in young people.
Its use in adolescence is consistently associated with poorer mental health outcomes including increased risk of mood disorders, self-harm and suicidality.
Its use is also associated with markedly poorer psychosocial outcomes across the lifespan in diverse indices such as educational attainment, employment, relationships, welfare dependency, risk of motor accidents, social mobility and income.
“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 study senior author Professor Mary Cannon, a researcher in the Department of Psychiatry at the Royal College of Surgeons and the Trinity College Institute of Neuroscience.
“Loss of IQ points early in life could have significant effects on performance in school and college and later employment prospects.”
For their systematic review and meta-analysis, the researchers searched Embase, PubMed and PsychInfo databases from inception to January 24, 2020.
They found seven cohort 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.
The young people were followed up until age 18 on average although one study followed the young people until age 38.
The authors found that young people who use cannabis frequently or dependently by age 18 have declined in IQ at follow up and this may be due to a decline in verbal IQ.
The findings could be explained by several potential mechanisms: a developmental neurotoxicity mechanism, a social pathway influenced by deviancy and educational non-engagement, by residual effects of cannabis or by individual vulnerability factors such as reading ability in childhood or by genetic factors.
Hypotheses such as family level vulnerability predisposing to IQ decline are possible but are less likely to be fully explanatory.
“Cannabis use during youth is of great concern as the developing brain may be particularly susceptible to harm during this period,” said study first author Dr. Emmet Power, a researcher in the Department of Psychiatry at the Royal College of Surgeons.
“The findings of this study help us to further understand this important public health issue.”
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Emmet Power et al. Intelligence quotient decline following frequent or dependent cannabis use in youth: a systematic review and meta-analysis of longitudinal studies. Psychological Medicine, published online January 27, 2021; doi: 10.1017/S0033291720005036
Source link: https://www.sci.news/medicine/frequent-cannabis-use-adolescence-lower-iq-09316.html
