A study undertaken in Dunedin found that adolescents who used cannabis and became dependent on it or used it regularly may suffer a decline in IQ later on in life but there were no significant changes in the IQ of participants who started using cannabis in adulthood. The researchers noted a number of limitations including that they could not rule out that their findings may be the result of an 'unknown variable'.
The experiment carried out by researchers at the University of Otago sought to test the popular belief held by adolescents that cannabis use was harmless and specifically focused on whether there is a prevalence of IQ decline in cannabis users who start smoking during adolescence. Note a potential bias: the study was founded by the ex-Chairman of DARE - a drug and alcohol resistance organisation targeted at adolescents and was funded by government institutions in both NZ and the US including other organisations whose objective was to reduce drug and alcohol abuse and organisations who sought to improve the wellbeing of children and young people. Arguably, the research may have been tailored to prove that cannabis use in adolescence was harmful.
The study group initially comprised of 1037 people born between 1972 and 1973 but in the last wave of the study at age 38 only 96% of the 1004 living study members re-tested. IQ tests were performed in childhood every 2 years from age 7 until 13, prior to the onset of cannabis use with the final test occurring when participants were 38 years old. The researchers conducted the study in waves and split the participants into groups dependent on their level of cannabis use or dependence. Researchers compared the IQ of participants showing persistent cannabis dependence (PCD)** and persistent cannabis use (PCU)*** to those who had never used cannabis. Use of cannabis in the day or week preceding the test is unlikely but the researchers admit this may not be accurate because the participants were not lab tested. Other controls used included hard drug or alcohol use, schizophrenia and education.
The study published in Proceedings of the National Academy of Science (PNAS) found that there was a statistically significant decline in neuropsychological function and mean IQ test scores in the PCD and PCU groups, which was not apparent in the never used group.
The never used group exhibited a positive effect size of 0.05 on average indicating an increase in mean IQ compared to the negative effect sizes of -0.38 and -0.35 on average for the PCD and PCU groups respectively corresponding to a decrease of about ~6 IQ points.
It can be estimated with 95% confidence that the never used group showed a mean difference in their full scale IQ on average somewhere between 85.39 and 115.89 IQ points, in comparison to the PCD group who showed a mean difference somewhere between 80.61 and 107.25 IQ points on average. The study identifies that those in the PCD group improved in the arithmetic, block design and picture completion subtests, the areas where the never used group showed decreases. This could possibly correlate to the results that show some PCD and PCU participants would have the same IQ as some of those who have never used cannabis despite the general declines measured overall.
The researchers recommend that policy decisions focus on delaying the onset of cannabis use in adolescence, and suggested funding further research to provide definitive results in light of limitations documented in the study.
Arguably, some further limitations might include the possibility of participants practicing IQ tests prior to testing, for instance, if the participants knew they might be IQ tested they might have wanted to improve their past IQ results. Additionally, occupation could also be a factor in the results, such that those who use higher-level problem solving and verbal comprehension skills on a daily basis – some of the main skills tested, may perform better on these tests in comparison those who have low skilled or unskilled roles or who are unemployed. The sampling frame was very narrow involving only participants from Dunedin, so with a larger and more diverse sample the results may differ. Finally, comparative studies with populations known to be persistent users such as those identifying as Rastafarian, could provide a valuable insight into the neuropsychological functioning of different ethnicities in response to cannabis use.
In my view there are multiple limitations in this study and I am therefore not convinced that it is cannabis use in adolescence which led to the results found by the study, but other variables not considered by the study. However, in saying that, the study does show that there is at least a risk that cannabis use in adolescence may lead to IQ decline later on in life. Results for adult onset cannabis use were insignificant leading researchers to conclude that the IQ decline was not prevalent among the adult onset group.
*The graph was composed based on the results in Table 1 of the actual study and is not an image from the study itself. The error bars in the graph indicate the standard deviation of the mean IQ scores on average.
**Persistent cannabis dependence was defined according to the number of waves at which participants met the cannabis dependence criteria. For the purpose of this article, the 3+ group are the focus.
***Persistent cannabis use was determined through regularity which was defined as 4 or more times per week. For the purpose of this article the 3+ group are the focus.
This information in this post was predominantly sourced from an assignment I completed for Uni that has yet to be graded.