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Study Finds ‘No Evidence’ That Medical Marijuana Causes Cognitive Impairment In Patients With Chronic Health Problems



Findings of a study examining marijuana’s neurocognitive effects “suggest that prescribed medical cannabis may have minimal acute impact on cognitive function among patients with chronic health conditions”—which may come as a relief to long-term cannabis patients who are concerned about potential neurological drawbacks of the drug.

Authors of the report, published last month in the peer-reviewed journal CNS Drugs, wrote that they found “no evidence for impaired cognitive function when comparing baseline with post-treatment scores.”

To conduct the study, researchers had 40 people in Australia self-administer a single dose of medical marijuana in a laboratory setting, following instructions on the product label. Participants were then tested on an array of neuropsychological metrics—including multitasking, pattern recognition memory, reaction time, rapid visual information processing and spatial working memory, among others—and surveyed on their subjective experience.

“The absence of evidence for cognitive impairment following medical cannabis self-administration was surprising,” the study says, “given prior and substantive evidence that non-medical (‘recreational’) cannabis use reliably impairs a range of cognitive functions. At the same time, these findings are consistent with two systematic reviews published in the last year that suggest that medical cannabis, when used regularly and consistently for a chronic health concern, may have little if any impact on cognitive function.”

“Medical cannabis, when used as prescribed, did not negatively impact cognitive function.”

Researchers also found “little evidence” between participants who consumed flower versus oil to suggest that “the magnitude of change in cognitive function differed with product type.”

Researchers did find some differences in effects between participants who consumed cannabis flower and concentrate, however. “Relative to participants who self-administered oil, those who self-administered flower rated themselves as more stoned” one hour after consumption, as well as more sedated, the study says. “Participants who self-administered flower rated themselves as more confident than those who self-administered oil,” it adds, “although this difference was only statistically significant at the 4-h timepoint.”

The report points out that cannabis tolerance increases with longer-term use, citing a 2021 study in which occasional users who were given a dose of THC “exhibited significant alternations in reward circuitry” as well as “impaired performance on a sustained attention task.”

“Chronic cannabis users administered the same dose of THC did not exhibit these same neurometabolic alterations or performance decrements,” authors wrote, “despite reporting a significant increase in intoxication relative to placebo.”

“Considering this emerging evidence for pharmacodynamic tolerance to the effects of cannabis,” they continued, “stable dosing with THC (and gradual dose titration up until, and not beyond, the point that effective symptom relief is achieved) is likely critical for any potential mitigation of impairment.”

The report advised people to take potential impairment into account, especially early on and after any increase in dosages.

“Performing safety-sensitive tasks, such as driving, is therefore not advised until patients are taking a stable dose of THC,” it says, “and should also be temporarily avoided following any increase in THC dose.

The lack of evidence for cognitive impairment may be due in part to the fact that participants were regular users of medical marijuana. Most (92.5 percent) had been using medical marijuana for more than a month before starting the trial, and all patients reported daily cannabis use.

Past studies that found impairment, by contrast, included “a requirement that patients avoid cannabis for 30 days prior to participating.”

“The fact that these patients [in past research] were not taking a stable dose of medical cannabis might explain why we found no evidence for acute cognitive impairment” in the current study, authors wrote.

In terms of how effective medical marijuana treatment was among participants, authors noted that patients self-reported a mean improvement in their condition of 78.6 percent since beginning cannabis treatment.

Among some of the study’s limiting factors were that it lacked a placebo, which authors said “means we were unable to disentangle the baseline performance from possible residual impairment resulting from medical cannabis use the evening prior.” Participants also weren’t screened for other drugs prior to the trial.

On top of that, they noted that the controlled use of medical marijuana might not match how patients use the drug in the real world. “For instance, patients may typically only use medical cannabis in the evening before sleeping,” the study says. “As patients had been using medical cannabis for more than 10 months on average, these findings cannot be generalized to patients who are just commencing treatment with medical cannabis or changing dose/product type.”

While the long-term effects of cannabis use are far from settled science, findings from a number of recent studies suggest some fears have been overblown.

A report published in April that drew on dispensary data, for instance, found that cancer patients reported being able to think more clearly when using medical marijuana. They also said it helped manage pain.

A separate study of teens and young adults at risk of developing psychotic disorders found that regular marijuana use over a two-year period did not trigger early onset of psychosis symptoms—contrary to the claims of prohibitionists who argue that cannabis causes mental illness. In fact, it was associated with modest improvements in cognitive functioning and reduced use of other medications.

“CHR youth who continuously used cannabis had higher neurocognition and social functioning over time, and decreased medication usage, relative to non-users,” authors of that study wrote. “Surprisingly, clinical symptoms improved over time despite the medication decreases.”

A separate study published by the American Medical Association (AMA) in January that looked at data from more than 63 million health insurance beneficiaries found that there’s “no statistically significant increase” in psychosis-related diagnoses in states that have legalized marijuana compared to those that continue to criminalize cannabis.

Studies from 2018, meanwhile, found that marijuana may actually increase working memory and that cannabis use doesn’t actually change the structure of the brain.

And, contrary to then-President Trump’s claim that marijuana makes people “lose IQ points,” the National Institute of Drug Abuse (NIDA) says the results of two longitudinal studies “did not support a causal relationship between marijuana use and IQ loss.”

Research has shown that people who use cannabis can see declines in verbal ability and general knowledge but that “those who would use in the future already had lower scores on these measures than those who would not use in the future, and no predictable difference was found between twins when one used marijuana and one did not.”

“This suggests that observed IQ declines, at least across adolescence, may be caused by shared familial factors (e.g., genetics, family environment), not by marijuana use itself,” NIDA concluded.

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Photo courtesy of Mike Latimer.

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Ben Adlin, a senior editor at Marijuana Moment, has been covering cannabis and other drug policy issues professionally since 2011. He was previously a senior news editor at Leafly, an associate editor at the Los Angeles Daily Journal and a Coro Fellow in Public Affairs. He lives in Washington State.


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