A study by Meier et al., entitled Persistent Cannabis Users Show Neurophysiological Decline from Childhood to Midlife, was published last year, at which time it received widespread media attention. But what did it actually find, and can its results be trusted?
Meier et al. followed around 1,000 individuals (comprising a single New Zealand birth cohort) for nearly 40 years, assessing their marijuana use at ages 18, 21, 26, 32 and 38, and their (age-adjusted) cognitive performance at ages 7, 9, 11, 13 and 38. According to the authors, there has only ever been one other cohort study of marijuana use and cognitive performance, namely Neurocognitive Consequences of Marijuana--A Comparison with Pre-drug Performance by Fried et al. (2004).
This older study observed marijuana users after a mean duration of usage of only 2 years, and had a sample size of only 121. Thus, Meier et al.'s study is by far the most comprehensive analysis of the subject to date. Before describing exactly what Meier et al. found, it was worth noting that--in spite of the small sample and limited duration of exposure after which users were assessed--Fried et al. documented significant differences in cognitive performance between heavy users (those smoking 5 or more joints per week) and controls. These differences (which were adjusted for prior performance, along with a number of other controls) are shown in the graph below. However, it is also important to note that Fried et al. did not find any significant differences between former users and controls.
As noted, Meier et al. assessed users' cognitive performance in childhood and then again at age 38. Their main finding was that cognitive performance between childhood and age 38 decreased in a roughly linear fashion according to the number occasions on which regular marijuana usage (4 or more joints per week) was reported. Users who never reported marijuana usage experienced a slight increase in cognitive performance between childhood and age 38, those who reported regular usage on only one occasion experienced a small decrease in cognitive performance, while those who reported regular usage on 3 or more occasions experienced a large decrease in cognitive performance. This trend is shown in the graph below (taken from the original paper); the scale on the horizontal axis is change in IQ (in standard deviation units).
Meier et al. observed the same pattern of decline on a variety of other measures of cognitive performance, and after controlling for sex and years of education. They also found a strong positive association between informant-reported memory problems (in the user) and number of episodes of regular marijuana usage. Interestingly, they observed a very small decline in cognitive performance among users who had begun smoking marijuana regularly after age 18, which suggests that regular marijuana usage may be more deleterious in adolescence than in adulthood. Finally, they observed a decline in cognitive performance even among (adolescent-onset) users who reported infrequent marijuana usage in the year before assessment at age 38, which constitutes weak evidence that cessation of usage may not yield full restoration of cognitive performance.
Despite having been released only a year ago, Meier et al.'s findings have already been called into question. In particular, Rogeberg (2013) published a paper entitled Correlations Between Cannabis Use and IQ Change in the Dunedin Cohort Are Consistent with Confounding from Socioeconomic Status. He makes the reasonable point that Meier et al.'s results are consistent with the idea that individuals predisposed to experience cognitive decline in adulthood select into marijuana usage. His conclusion is that the decline in cognitive performance among marijuana users observed by Meier et al. is almost certainly an over-estimate of the causal effect of marijuana usage.
Overall, there seems to be reasonably good evidence that smoking marijuana lowers cognitive performance in the short-term. However, the evidence that smoking marijuana permanently lowers cognitive performance is, at the present time, still relatively weak.