Subject:
Substance Abuse

Short Description:
Marijuana May Hurt Cognitive Function, Worse with Heavier Use

Background:

Does marijuana affect cognitive function? There seems to be no shortage of opinions on this question, but no prospective, longitudinal study has ever been published. Until now, that is. Researchers in New Zealand followed more than 1,000 subjects from their birth in 1972–1973 until age 38, to ask whether regular cannabis users showed any evidence of cognitive decline.

The answer, surprisingly (or not, depending on where you stand on the issue), was a resounding “yes.” Studying the 1,037 individuals in the famous Dunedin Study, researchers found that those who were dependent on cannabis, or even who used it regularly, experienced an overall decrease in IQ. Moreover, the extent of decline was correlated with heavier cannabis use. Those who admitted to regular cannabis use on more than three follow-up assessments in the 38-year period, for example, lost an average of six IQ points (99.68 to 93.93) between age 13 and age 38, while those who reported it only once lost only one point. Non-users actually gained an IQ point. Decline was apparent in multiple cognitive domains, including executive function, memory, processing speed, and verbal comprehension, and again, performance was worse with heavier use.

The researchers performed multiple controls. They eliminated those with other drug or alcohol dependence, or schizophrenia, and they even controlled for years of education. With each analysis, the same trend held true, and was statistically significant. They even interviewed informants (people “who knew the subjects well”), and these independent observers noticed more attention and memory problems among subjects with heavier cannabis use. Perhaps most notably, the subset of users who were diagnosed with cannabis dependence before age 18 lost IQ points (as many as eight), while adult-onset cannabis users experienced no such decline (Meier MH et al, Proc Natl Acad Sci USA 2012, online ahead of print).

TCPR's Take:
This was an impressive study, not only for the consistency of its findings, but for its scope, involving more than 1,000 subjects followed at regular intervals over 38 years. The study may have been limited by underreporting of drug use or the lack of objective drug-use data (like urine samples), and it should not be interpreted as proof that cannabis use causes cognitive impairment. Nonetheless, the observations that IQ loss was most concentrated in those who started using in adolescence, and the extent of loss was correlated with heavier cannabis use, argue in favor of potentially detrimental effects of cannabis on the developing brain. If you’re looking for evidence to convince your younger patients that marijuana use may predict lower IQ in adulthood, look no further.