Lifetime cannabis use does not appear to negatively impact cognitive decline, and may actually protect against it, according to a longitudinal study published in Brain and Behavior.
The research assessed the relationship between marijuana use and age-related cognitive changes in a cohort of 5,162 Danish men over a span of 44 years.
Participants’ IQs were evaluated in early adulthood, with a mean age of 22, and again in late midlife, at an average age of 62. The study found that men with a history of cannabis use experienced “significantly less cognitive decline” over their lifetimes compared to non-users.
According to the researchers, the average cognitive decline among all participants was 6.2 IQ points. However, cannabis users showed a decline that was 1.3 IQ points less than non-users, even after adjusting for potential confounding factors. Neither the age at which cannabis use began nor the frequency of use had a measurable negative impact on cognitive decline among users.
“Men with a history of cannabis use had less cognitive decline from early adulthood to late midlife compared to men without a history of cannabis use”, states the study. “Among cannabis users, neither age of initiation of cannabis use nor frequent use was significantly associated with a greater age-related cognitive decline.”
The study’s authors concluded that their findings align with most existing research, which generally suggests no strong association between cannabis use and cognitive decline. They noted, however, that further research is needed to clarify whether these results indicate the absence of long-term adverse effects or if the effects of cannabis on cognition are temporary and dissipate over time.
This study challenges long-standing claims that marijuana use harms IQ or cognitive abilities over time. Critics of earlier studies alleging a link between cannabis and lower IQ have pointed out flaws in their methodologies, including failures to account for socioeconomic factors. For example, a widely publicized 2012 study suggesting early cannabis use led to lower midlife IQ was later criticized as methodologically flawed.
NORML’s Deputy Director Paul Armentano commented on the findings, stating, “These results contradict one of the more prominent and longstanding stereotypes about cannabis and cannabis consumers. It is unfortunate that studies refuting these stereotypes rarely receive the attention they deserve.”
Additional research has similarly failed to demonstrate significant adverse cognitive effects from cannabis use. A review published in JAMA Psychiatry earlier this year reported that while short-term deficits in cognition may occur with cannabis use, these effects diminish after periods of abstinence longer than 72 hours.
