Cannabis use not associated with IQ decline, according to recent study

Cannabis use has long been associated with negative impacts on IQ. A recent longitudinal study published in Brain and Behavior reveals that lifetime cannabis consumption is not not associated with lower IQ.

Key study findings

The Danish study analyzed a cohort of 5,162 men over a 44-year period. Participants' IQ levels were assessed first in early adulthood (mean age: 22), then at the end of middle life (mean age: 62). The researchers found that cannabis users had «significantly less cognitive decline »In other words, they are more likely than non-consumers to experience a "lifetime of consumption".

Specifically, users had a lower average cognitive decline than non-users:

• Non-users experienced an average cognitive decline of 6.2 IQ points over the period studied.
• Cannabis users experienced an average decline of 1.3 IQ points than non-consumers.

The study also examined the impact of variables such as’age of cannabis initiation and the consumption frequency. These factors were not correlated with adverse cognitive outcomes, suggesting that the duration or onset of use had no significant effect on IQ.

The authors concluded that «years of frequent cannabis use were not generally not associated with a significant difference in cognitive decline compared with no frequent use. Further studies are needed to determine whether these results reflect the absence of negative effects on cognitive decline, or whether the effects of cannabis are temporary and disappear after a prolonged period of time».

Breaking the cannabis stereotype

These results directly contradict the long-held belief that cannabis use inevitably leads to cognitive impairment. Other research has also refuted claims that cannabis use impairs IQ :

1. A British study involving more than 2,000 adolescents found no correlation between early exposure to cannabis (before age 15) and lower IQ or academic performance, after taking confounding factors into account.
2. The twin studies failed to establish a causal link between cannabis use and cognitive impairment
3. A JAMA Psychiatry literature review reported that cognitive deficits observed in cannabis users were minimal and often disappeared after more than 72 hours of abstinence.

While a 2012 study suggested that early cannabis use could lower IQ in midlife, its methodology was subsequently criticized. According to critics, the study failed to take into account socio-economic factors, which probably skewed his conclusions. A published response in the Proceedings of the National Academy of Sciences stated that «the causal effects estimated [by the study authors] are probably overestimated, and the true effect [of cannabis on IQ over the life course] may be zero».

In addition, other longitudinal research, including recent published data in the Journal of the American Medical Association, The results of a study conducted in the United States on the use of cannabis for medical purposes revealed no significant negative effects of cannabis use on brain morphology or cognitive performance in adult patients.