By Jim Oldfield
Cannabis has long been linked with memory loss. From Dude, Where’s My Car? and other weed films, to the yellow warning labels on legal cannabis products and a strong body of research that shows cannabis impairs recall — the idea that pot disrupts memory is firmly lodged in the public mind.
Or it was, at least. Collective concern about the link between cannabis and memory may be fading as governments legalize recreational access to cannabis. Their aims include encouraging more responsible use, regulating the supply of cannabis products and destigmatizing historically marginalized groups.
In Canada, five years after the legalization of recreational cannabis, many experts see some progress toward those goals. But at the same time, more Canadians are using the drug, highly potent products have proliferated, and public awareness of the harms of cannabis is sketchy.
“Many Canadians see the negative effects of cannabis on memory and other aspects of health as collateral damage, if they’re aware of them at all,” says Tony George, a psychiatry professor at the Temerty Faculty of Medicine and a clinician-scientist at the Centre for Addiction and Mental Health (CAMH), who has studied substance use and addiction for decades.
Memory is an essential brain function because many other forms of cognition depend on it, says George. Memory tests are standard in most cognitive assessments, and issues with memory can signal everything from minor health problems to major neurodegeneration.
Many studies have found that cannabis affects short-term memory. In 2021, George and colleagues published a systematic review of cannabis research in the previous 30 years. It showed that in the majority of studies, cannabis use among healthy people was associated with impairments in verbal, episodic and working memory.
Another review by George’s lab found that the negative effects on memory were more pronounced in people with existing conditions, especially schizophrenia and depression. This finding dovetails with clinical studies by George’s team, which show that abstinence from cannabis in people with depression and other disorders can positively affect memory.
These results underscore the risk cannabis poses to vulnerable people — often forgotten in public debate on legalization — and highlight long-standing inconsistencies in cannabis research. “There hasn’t been much systematic, rigorous study of cannabis,” says George. “Study populations vary, as do doses and routes of administration.”
The upshot is that some key questions on cannabis’ effect on memory are unanswered, including how long impairments to short-term memory last, and to what degree. This uncertainty could well give Canadians pause, but instead, legalization has coincided with more people using the drug.
The number of Canadians who consumed cannabis grew six per cent between 2017 and 2020, to about one in five, according to a new report from Statistics Canada. That trend had been evident for decades, but it continued across most age groups after legalization and remained highest in those aged 18 to 24, who also reported the most frequent use.
The numbers in youth aged 15 to 17 were stable, but a trend toward fewer users in that group stopped after legalization. “The government likely hoped to see fewer adolescents using cannabis, since protecting them was one goal of making it legal,” says Justin Matheson (PhD ’20), a postdoctoral fellow in the lab of Bernard Le Foll, a pharmacology and toxicology professor at Temerty Medicine and a clinician-scientist at CAMH.
Studies have raised concerns about the effect of cannabis on brain development in young people. But Matheson says the post-legalization surge in high-potency products, readily available in retail stores and embraced by young users, is also worrying. Levels of tetrahydrocannabinol (THC), the main psychoactive compound in cannabis, now average 15 per cent in dried weed — a five-fold increase from past decades.
“Potency is a huge concern,” says Matheson, who studies cannabis use and its health effects. “A lot of dried cannabis is now 30 per cent THC.
I was young and didn’t care . . . cannabis made me happy
With edibles and oils, some of which reach 90 per cent THC, it’s easy to get massive doses.” The long-term effects of stronger cannabis on memory are unknown, but Matheson says acute memory impairment is clearly, if not neatly, dose related.
A Toronto woman who quit long-term cannabis use in 2021 after being treated at CAMH, says her memory was worse during periods of heavy use. “I’ve always been forgetful, but day-to-day stuff was definitely harder to remember and keep organized,” says the 27-year-old, who chose not to be identified.
For a good high, she recalls needing increasingly stronger doses of THC, sometimes every hour, but her memory of those times is hazy. “They seem like a dream. I know certain things happened, but it’s like they happened to someone else,” says the woman, who later returned to school and is now a nanny and personal support worker.
The woman was aware of some of the risks that cannabis poses to memory and health, but says she ignored them. “I was young and didn’t care. And essentially, I was often unhappy. Cannabis made me happy.”
Herein lies another challenge to Canada’s cannabis framework: how to raise public awareness and make cannabis education more effective.
Before legalization, many Canadians held benign views on cannabis and safety. Only 58 per cent of respondents in the 2017 Canadian Cannabis Survey said that the drug had a negative effect on memory, for example. And only half of those who had used cannabis in the previous year said it affects driving ability.
Since legalization, most data show that Canadians are increasingly aware of the potential harms from cannabis, but progress has been slow. And some other signs are worrying. In the 2021 Canadian Cannabis Survey, far fewer people reported seeing warning messages on cannabis products or noticing education campaigns.
Moreover, gaps exist in the cannabis research that should complement those public health efforts. “Research has not accelerated the way we hoped it would after legalization,” says Ruth Ross, a pharmacology and toxicology professor at Temerty Medicine and the director of the Toronto Cannabis and Cannabinoid Research Consortium.
Ross says that while bureaucratic regulations have hindered research, the low funding levels for research and cannabis education are a bigger issue. Federally, the government committed $108 million for public health messaging on cannabis from 2017 to 2023. But it’s fair to ask if that’s enough, given that the gross domestic product of Canada’s cannabis sector is over $10 billion annually.
Provincially, the allocation of cannabis tax revenues has varied. The Ontario Cannabis Store put only 0.1 per cent of its $170 million in net income to a social impact fund last fiscal year, Ross noted in a recent opinion piece. In contrast, Quebec put all $95 million in net earnings toward its cannabis prevention and research fund.
Ross says that since the legalization of cannabis, emerging data suggest that cannabis-related harms are lower in Quebec than in other provinces. “We’re facing major challenges with our model of legalization, but we could still get it right, and Quebec could be a guide,” she says. “We need to be flexible.”
Canadians may not have forgotten that cannabis affects memory, but we might do well to remember what we know — and don’t know — about cannabis and its harms. •