Individuals using cannabis on a daily basis before the age of 17 years are significantly less likely to complete high school and obtain a degree and significantly more likely to be cannabis dependent, use other illicit drugs, and attempt suicide by age 30 years, results of a metaanalysis have shown.
Researchers combined and analyzed individual patient data from three large longitudinal studies involving a total of 3,765 participants, finding that those who smoked cannabis on a daily basis when young were 63% less likely to complete high school and 62% less likely to attain a degree, compared with never-smokers.
"Furthermore, daily users had odds of later cannabis dependence that were 18 times higher, odds of use of other illicit drugs that were eight times higher, and odds of suicide attempt that were seven times higher," wrote Edmund Silins, Ph.D., from the Drug and Alcohol Research Centre, University of New South Wales, Sydney, and his colleagues.
The researchers also found clear evidence of a dose-response association between the intensity of cannabis use and the degree of impact on later outcomes, even after accounting for potential confounders such as age, sex, ethnicity, socioeconomic status, and mental illness, according to Dr. Silins and his associates (Lancet Psychiatry 2014 Sept. 9).
Participants were categorized into five groups according to the intensity of cannabis use: never-users, less than monthly, monthly or more, weekly or more, and daily users, with never-users serving as the control group.
"The unique thing about this study is that up to now, existing studies are pretty limited in statistical power to examine heavy cannabis use specifically in younger groups, because there aren’t that many heavy users before the age of 17," he said in an interview. "We focused on the results for daily users because they’re the most extreme, and also the most important to consider because they’re the group that are likely to keep using and also have problems later."
After they adjusted for confounders, the investigators did not find an association between daily cannabis use and either welfare dependence or depression.
The authors said that the findings were particularly relevant, given movements toward the legalization of cannabis in several countries.
"The findings strongly suggest any reforms to cannabis legislation should be carefully evaluated to make sure that they would reduce adolescent cannabis use and prevent any potentially adverse effects on adolescent development," Dr. Silins said.
The investigators cited several limitations. For example, the data were obtained by self-report, "which might be subject to socially desirable response bias, the extent of which can vary with age," they wrote.
In an accompanying editorial, Dr. Merete Nordentoft of the Mental Health Center Copenhagen said that there was some debate about whether the link between cannabis and low educational attainment was influenced by socioeconomic group, whether cannabis use may be a marker of already-established development disadvantage, or whether it was a truly causal association.
"[This] report, which is based on participant-level data from three studies from Australia and New Zealand, clearly shows a dose-response association between frequency of cannabis use in adolescence and ability to complete specific tasks that mark the transition to adulthood, specifically the ability to achieve high levels of education," Dr. Nordentoft wrote.
The study was funded by the Australian Government National Health and Medical Research Council. No other conflicts of interest were declared.