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Researchers have issued a warning about cannabis use as the drug becomes legal in more places.
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  • In a new study, researchers say cannabis users can have cognitive impairments beyond the initial use period.
  • The cognitive impairments can affect decision making, memory, and the ability to complete mental tasks.
  • Some experts say the research is flawed because it doesn’t take into account the different types of cannabis and the different ways it can be ingested.
  • Some professionals who treat cannabis-related ailments say they have seen the effects of the drug’s use, especially in younger people whose brains are still developing.

People using cannabis can have acute cognitive impairments lasting beyond the period of intoxication, according to a study led by Canadian researchers that was published today.

The researchers say it’s important for users to understand these long-term effects as cannabis legalization becomes more common.

The researchers looked at the effects of cannabis on more than 43,000 participants.

They reported that cannabis causes small to moderate cognitive impairments in areas that impact decision making, suppressing inappropriate responses, learning, and remembering through listening and reading as well as how much time someone needs to complete mental tasks.

The study suggests that the negative health effects of cannabis, which begin during consumption, can last for a significant amount of time.

“Our study enabled us to highlight several areas of cognition impaired by cannabis use, including problems concentrating and difficulties remembering and learning, which may have considerable impact on users’ daily lives,” wrote Dr. Alexandre Dumais, an associate clinical professor of psychiatry at Université de Montréal and a study co-author.

“Cannabis use in youth may consequently lead to reduced educational attainment, and, in adults, to poor work performance and dangerous driving. These consequences may be worse in regular and heavy users,” he said.

The researchers issued a warning about growing acceptability of cannabis being “on the rise,” especially among young people, who have the highest rates of use.

“It is therefore important to understand the cognitive risks involved in using cannabis, especially to young people, whose brains are undergoing significant developmental changes,” the study authors wrote.

Experts disagree on the extent of the negative effects caused by cannabis. Research into cannabis’ effects has been somewhat limited, due to it being illegal and not available for many research projects.

“Marijuana can be harmful to the brain and it can also be useful for medical purposes. The reality is that there is just not enough research to know a precise answer,” said Dr. Rebecca Siegel, a New York City clinical psychiatrist and author of “The Brain on Cannabis: What You Should Know About Recreational and Medical Marijuana.”

“We do know that cannabis use can impact the brain’s frontal lobe which controls executive functioning — decision making, problem-solving, etc.,” Siegel told Healthline. “Because THC affects the hippocampus and prefrontal cortex, marijuana users may experience difficulty with concentration and memory. But without those studies, we can’t know for sure and we won’t know until more uniform, controlled studies are done.

“We do know that the brains of young adults are continuing to form until their mid-20s, so cannabis use in that population needs to be carefully controlled and monitored by a physician,” she said.

Michele Ross has a PhD in neuroscience and has studied the effects of cannabis on the brain for the National Institute on Drug Abuse.

She told Healthline the study is flawed because of its limitations and that it ignores millions of productive people all over the world who regularly use cannabis.

“The studies likely don’t look at cannabis consumed in other ways, including cannabis tinctures, cannabis edibles, etc., which don’t expose the user to harmful effects of smoke, which cause memory problems themselves,” Ross said. “They also don’t look at smoking cannabis that includes both THC and CBD, which has been shown to reduce any potential temporary harms of smoking high-THC cannabis strains.”

Dr. Adam D. Scioli, medical director and head of psychiatry at Caron Treatment Centers, told Healthline the way cannabis is consumed doesn’t matter when it comes to its potential damage.

“It doesn’t matter how you take cannabis because the long-term intoxicating and impairing effects are similar,” Scioli said. “Factors that could impact impairment include the concentration of THC in the cannabis, quantity, frequency, and duration of exposure.

“Genetics, as well as underlying medical and mental health disorders, also contribute to the severity of cognitive impairment in individuals exposed to cannabis,” he said.

Scioli said he sees firsthand confirmation of the study’s conclusions.

“The findings confirm what we’ve discovered empirically through psychometric testing in adolescent and adult patients at Caron who struggle with cannabis use disorders,” Scioli said. “We see moderate to severe impairment in executive functioning that is consistent with the cognitive impairment documented in this study.

Scioli said there’s no easy way to erase the negative effects of cannabis.

“The best approach is not to use cannabis or seek help if you’re using and struggling to stop on your own,” he said. “Oftentimes with cessation, we observe gradual improvement in cognitive function. But that’s not always the case. This research is critical because it acts as a cautionary tale and highlights the need to slow down and better understand the long-term implications of cannabis use instead of making it a magic elixir to fix everything.”