Scientists from the Harvard Medical School and from the intramural research program of the National Institute on Drug Abuse (NIDA) found lasting cognitive deficits in those who started to smoke marijuana before age 17. The researchers analyzed neuropsychological test results from 122 long-term heavy users of marijuana and 87 subjects who had used marijuana only a few times (control subjects).
Marijuana Affects Verbal FunctionSixty-nine of the 122 users started using marijuana at age 17 or before. The subjects were between the ages of 30 and 55 at the time of the study, and all had refrained from any drug use 28 days prior to testing.
Individuals who started using marijuana at age 17 or younger performed significantly worse on the tests assessing verbal functions such as verbal IQ and memory of word lists than did those who started using marijuana later in life or who had used the drug sparingly. There were virtually no differences in test results among the individuals who started marijuana use after age 17 and the control subjects.
Lower Cognitive SkillsThe investigators suggest three possible hypotheses that might explain these differences. One possibility is that early-onset smokers had lower innate cognitive skills before they ever started smoking marijuana. A second possibility is poor learning of certain cognitive skills by young users of marijuana who neglect school and academic pursuits.
The third and most ominous possibility is that marijuana itself has a neurotoxic effect on the developing brain. According to the authors, further research will be required to determine the relative contributions of these three factors. Dr. Harrison Pope and colleagues published the study in the March 2003 issue of the journal Drug and Alcohol Dependence.
Pope, HG, et al. "Early-onset cannabis use and cognitive deficits: what is the nature of the association?" Drug and Alcohol Dependence 1 April 2003.