Long-Term Marijuana Use May Not Lead to Lasting Brain Changes, Study Suggests

For years, marijuana users have faced warnings about its potential to harm brain function, particularly memory and cognition. While it’s true that marijuana can temporarily impact short-term memory—making it harder to retain and recall new information—many have worried that long-term use might lead to irreversible cognitive decline. A new, decades-long study out of Denmark provides reassuring news: long-term marijuana use does not appear to accelerate age-related cognitive decline.

The research, conducted by scientists at the University of Copenhagen, followed over 5,000 men for more than 35 years. Using standardized intelligence tests administered during mandatory military conscription between the ages of 18 and 26, participants were reassessed at around age 64 to measure cognitive changes over time.

Participants were also asked about their history of marijuana use. About 40% of the men reported trying weed at least once, while roughly 10% identified as frequent users (at least twice a week) for less than 10 years. Another 12% reported frequent marijuana use lasting a decade or longer.

When researchers compared IQ declines between marijuana users and non-users—while controlling for other factors like education, smoking, and alcohol use—they found something surprising: weed users actually showed slightly less cognitive decline over time. On average, their IQ scores dropped 1.3 points less than those of non-users.

While the difference is small and may not be clinically significant, the findings provide comfort to long-term marijuana users who feared permanent brain changes.

These results add to a growing body of evidence that marijuana’s cognitive effects may be temporary, rather than permanent. Previous studies have shown that any cognitive impairments caused by frequent weed use can reverse after sustained abstinence. For instance, research has found that former heavy users often recover cognitive function within a few months of stopping marijuana.

This Danish study’s findings align with earlier research. A 2016 study in Australia found no link between marijuana use and cognitive decline in middle-aged adults, men or women. The results also suggest that marijuana may not share the same long-term risks as substances like alcohol or tobacco, both of which are strongly associated with faster cognitive aging.

What’s particularly notable is that 92% of marijuana users in the Danish study had abstained from weed for at least a year prior to the follow-up IQ test. This highlights a critical point: even long-term, frequent marijuana users may not experience permanent cognitive harm, especially if they stop or reduce their use later in life.

While the study’s participants were exclusively men, its results provide reassurance to many long-term weed users around the world. The fear of irreversible brain changes has been a significant source of anxiety for those who have used marijuana regularly, particularly during earlier years of prohibition when stigma was high and information was scarce.

The researchers emphasize that more studies are needed, particularly among women and those who continue heavy marijuana use later in life. However, this study offers a much-needed counterpoint to decades of fear-based messaging.

For long-term marijuana users who have worried about the state of their brain health, this research brings welcome relief: marijuana use, even over many years, may not lead to permanent cognitive decline. And for those who have chosen to step away from weed, the evidence suggests that any cognitive effects are likely reversible with time.