Over the past two decades, the effects of marijuana use on the brain, on psychological function, social outcomes, and general health have been revealed as never before. The void of information in the preceding decades is now filling rapidly. The dramatic rise in findings is driven by long-term studies that follow people from early adolescence into adulthood, by rigorous new methods for evaluating behavioral and cognitive consequences, and by new brain imaging techniques that have begun to show brain changes that were inaccessible 20 to 40 years ago. The research has yielded significant evidence that use of marijuana, especially among the most vulnerable in our population—young people—is associated with a range of adverse consequences, including symptoms of psychosis, impaired cognition, addiction, motor vehicle accidents, reduced educational achievement, increased reliance on public assistance, and increased risk for pulmonary and cardiovascular problems.
As many of these problems are correlated with age of onset of use and cumulative amount of marijuana use, there is a growing consensus that marijuana conceivably is the causative agent. By far the most compelling evidence arises from longitudinal studies in which people are recruited and monitored prior to using the drug and then followed for long periods of time, even decades.
One recent longitudinal study has probably received more recent press on marijuana’s hazards than any other. The study, reported by Dr. Madeline Meier et al. (Proceedings of the National Academy of Sciences, 2012), showed that marijuana use initiated in early teens, is linked to a significant drop in IQ, when measured at age 38 in persistent users. The study was initiated with 1,037 early teenagers (13 years old), born from 1972 to 1973 in Dunedin, New Zealand. IQ and cognitive testing was measured in all the children who volunteered for the study, before they had started to use marijuana. Later on, they were questioned on marijuana use at ages 18, 21, 26, 32, 38. At age 38, IQ and cognitive testing were once again measured, along with questions of their marijuana use.
The study questioned six issues:
- Does persistent marijuana use from childhood to adulthood lead to cognitive decline?
- Does marijuana use lead to specific impairment or does it compromise global impairment (e.g., IQ)?
- Are there confounding effects that compromise cognitive function and not marijuana: for example, acute or residual marijuana intoxication, tobacco, alcohol, drug addiction, (e.g., heroin, cocaine, amphetamines), or schizophrenia, years of education? The latter was an important issue, as staying in school can enhance IQ and marijuana users tend to receive less schooling.
- Did marijuana use interfere with users’ daily lives? Contacts of the subjects were asked whether they perceived that marijuana had interfered with the users’ daily life.
- Was age on onset of marijuana use a critical factor? It was postulated that the earlier the age of initiation of marijuana use, the more likely the effects of marijuana.
- Was a reduction in IQ and function reversible, depending on age of quitting or reducing use?
All subjects were of similar, average IQ, 100, at the beginning of the study. Marijuana use initiated in early teens was linked to a significant drop in IQ, when measured at age 38 in persistent users. The findings were interpreted to suggest that marijuana may produce a neurotoxic effect on the adolescent brain. The most severe loss of IQ was 8 points, reducing the IQ score from 100 to close to 92. This suggests that the individual whose IQ started at 100, in the 70% percentile of the general population for intelligence, fell to 30% of the population in intelligence.
The effects were global and could not be explained by use of or addiction to other substances, by educational achievement of presence of psychosis. Marijuana use interfered with users’ daily lives, as interviewed personal contacts of 38-year-old subjects with a marijuana use disorder reported noticing more cognitive problems in the subjects. Impairment was highest among adolescent-onset marijuana users; the greater and more persistent the marijuana use, the greater the loss of cognitive function. Stopping marijuana use did not fully restore neuropsychological functioning among adolescent-onset users.
Criticism of the study emerged, notably a Norwegian Dr. Ole Rogeberg who challenged the study on the basis that socio-economic factors were responsible for the loss in brain function. Dr. Meier responded with a further analysis of the data, interrogating whether socio-economic factors may have played a role. The analysis failed to show that socio-economic factors were a major contributor to the findings.
The power of Meier study is its long term follow-up of similarly matched adolescents. The major factor that stood the test of statistical significance was their lifetime pattern of marijuana use. Other factors, for example educational achievement, alcohol consumption, were factored in and found not to change the conclusions of the findings. The limitations of human studies (accurate self-reporting and recall of drug use patterns) can be improved with parallel animal studies, and these have borne out some of the conclusions. Animals exposed to cannabinoids during adolescent development manifest changes in the brain’s reward system, and in the brain region critical for learning and memory.
In summary, the study showed that persistent marijuana use over 20 years duration was associated with psychological decline, with decline greater with longer marijuana use and adolescent onset. Among adolescent onset users who had used persistently but then quit, impairment was still quantifiable after stopping use for 1 year or longer. It also ruled out deficits that occurred prior to marijuana use, as all the subjects were of similar IQ at the onset of the study. The impairment was not discrete, but extended to five domains of psychological functioning. This association does not prove unequivocally that marijuana is causal.
More recently, Filbey et al. (Proceedings of the National Academy of Sciences, November 2014) combined brain imaging and psychological testing to test a similar hypothesis. This cross-sectional study (comparing marijuana users to non-users but not over the course of 25 years) found a 5-point lower IQ in the average marijuana user compared with the non-user. Once again they confirmed that the earlier the subjects had begun using the drug, the worse the brain changes. This study adds to a growing body of evidence that marijuana is harmful to young brains. A different study questioned whether marijuana use as a function of age of onset and amount of use, was related to socio-economic factors: the lower the age of marijuana initiation and the higher the amount of lifetime marijuana use was associated with dependence on welfare, unemployment, reduced income level and lower rates of earning a university degree.
What are the implications of these findings?
The adolescent brain is not fully developed and introduction of marijuana could interfere with normal brain development. As regular marijuana use during adolescence can lead to enduring effects on brain function, including a loss of IQ and educational achievement, it is critical to inform adolescents and others on the potential adverse effects of marijuana. Whether this loss is sufficient to interfere with daily function was not directly tested in subjects, but interviews with their contacts implied that they appeared to be cognitively impaired.
Beyond being linked to reduced IQ, education, income, and increased unemployment, teen marijuana use is linked to school dropout, other drug use, mental health problems, higher prevalence of addiction, and others. Given unprecedented numbers of teens who use marijuana daily or almost daily, and the possibility of this cohort increasing with marijuana legalization, it is critical for the public to recognize that regular marijuana use can affect the future of adolescent users for decades.