Getting high on cannabis is known to temporarily impact a person's memory, making it harder to retain and recall new information. But how the brain copes with cannabis in the long run is less clear.
A decades-long study of more than 5,000 men in Denmark has now uncovered "no significant harmful effects of cannabis use on age-related cognitive decline."
The findings stand in contrast to what has been found for a history of tobacco or excessive alcohol use, both of which are associated with faster cognitive decline. While more research is needed, especially among heavy, continuous cannabis users, this initial data is a positive sign.
In the new study, researchers at the University of Copenhagen measured cognitive decline using Denmark's conscription board intelligence test, which is mandatory for men between the ages of 18 and 26. A total of 5,162 participants retook the test at around age 64, between 35 and 53 years after their first go.
At the follow-up, participants were asked if they currently used or had ever used cannabis. About 40 percent reported trying cannabis at least once. Roughly 10 percent said they were frequent users – at least twice a week – for less than a decade. Nearly 12 percent were frequent cannabis users for a decade or longer.
Even when accounting for other factors that can contribute to lower IQ scores, like education level, tobacco smoking, or binge drinking, models revealed significantly less cognitive decline among cannabis users compared to nonusers.
Specifically, the IQ decline for cannabis users was 1.3 points less.
That's a modest difference that "may not hold clinical significance," the authors caution. However, recent studies have found that low doses of THC can show protective effects on the brain function of older mice.
In humans, early research suggests cannabis might even boost higher-order brain functions, or protect against some neurodegenerative diseases, like Alzheimer's.
"Among cannabis users, no significant association was found with cognitive decline for either age of initiation of cannabis use or frequent cannabis use," explain the authors of the current study, led by public health scientist Kirstine Maarup Høeg.
"Further studies are needed to investigate whether these findings reflect that there are no adverse effects on cognitive decline or that the effects of cannabis are temporary and disappear after a prolonged period of time."
The data from Denmark only considers men, but the results agree with a few other studies from elsewhere in the world.
A 2016 study from Australia, for instance, found no link between cognitive decline and cannabis use in middle-aged men or women.
Other analyses, however, have shown a link between cannabis use and cognitive decline, although these studies considered current users as opposed to past users.
In the Danish study, about 92 percent of cannabis users had not used cannabis in the year before the follow-up IQ test, so the results may not apply to those who continue using cannabis frequently in later life.
"Several studies suggest that the negative effects of cannabis on cognitive functions can be reversed with prolonged abstinence," the researchers explain.
"Adverse effects were not apparent after three months of sustained abstinence, even in former heavy users, indicating that frequent cannabis use may not cause irreversible damage."
Cannabis has become one of the most popular recreational drugs in the world, and yet because of its legal status in many nations around the world, scientists know very little about the plant's long-term health effects.
Studies like this one can help inform and educate users on some of the effects they might face from cannabis down the road.
The study was published in Brain and Behavior.