Marijuana health myths debunked

By Naveed Saleh, MD, MS
Published August 21, 2020

Key Takeaways

Marijuana is the most commonly used recreational drug among Americans, with an estimated 50% of the population partaking (par-toking?). Despite widespread use, little is known about its long-term effects, and myths abound.

Some experts believe that marijuana affects intelligence quotient (IQ). Although certain studies have suggested that marijuana use could harm the adolescent brain, including measures of IQ, because it’s unethical to study any causation between marijuana and declines in IQ, it is difficult to determine whether marijuana or confounding variables are to blame for drops in IQ.

In addition to harming the adolescent brain, some experts have suggested that marijuana legalization will increase marijuana use among teens. They have argued that state laws allowing marijuana for medical purposes could send a message that marijuana is acceptable among adolescents

Another suggestion is that marijuana use will decrease the risk of opioid overdoses. This hypothesis stems from the potential of cannabis to exert an opioid-sparing effect.

The following is an evidence-based deconstruction of these claims.

Marijuana and IQ

Based on results from two cohorts involving twins (n=789 and n=2,277) and published in the Proceedings of the National Academy of Sciences (PNAS)—a “quasi-experimental approach” per the authors—declines in adolescent IQ may not be due to marijuana use, but rather familial factors that set the stage for marijuana initiation and decreased intellectual attainment. 

“There are many potential variables related to adolescent marijuana use that could be confounding its relationship with IQ. Although some of the more salient factors (e.g., socioeconomic background and ethnicity) were explicitly measured in the present study, it would be impossible to identify and/or operationalize all relevant factors,” the authors wrote. 

They continued, “The co-twin control design is advantageous because it allows researchers to account for any unmeasured influences experienced in common by members of the same twin pair.”

In the study, researchers administered standardized measures of intelligence before marijuana use, at ages 9–12 years, and then again at ages 17–20 years. Participants self-reported marijuana use at the time of each cognitive assessment, as well as during the interim. The researchers accounted for family background characteristics and genetic propensities.

They found that by late adolescence, marijuana users experienced greater IQ declines in vocabulary, verbal ability, and general knowledge compared with those measures in nonusers. Baseline IQ was measured in a mostly White cohort aged 11-12 years from Minnesota and in a mostly Hispanic cohort from Southern California. In the Minnesota cohort, baseline assessments, which were taken before marijuana use began, indicated that users already had lower scores on intelligence measures versus nonusers. The statistically significant decrease in vocabulary scores equated to a change of nearly 4 IQ points in both samples.

However, results did not indicate a dose-response relationship between frequency of marijuana use and IQ change. Moreover, marijuana-using twins did not exhibit significantly greater IQ declines compared with their abstinent siblings.

“Evidence from these two samples suggests that observed declines in measured IQ may not be a direct result of marijuana exposure but rather attributable to familial factors that underlie both marijuana initiation and low intellectual attainment,” the authors noted.

Although marijuana use may not be detrimental to IQ, previous research indicated that its use does come at a high societal cost in terms of absenteeism, lower productivity, unemployment, and increased rates of crime/incarceration. Furthermore, some data have indicated that in terms of neurocognitive function, the adolescent brain may be most vulnerable. In fact, marijuana use has been linked to decreased memory, lower verbal ability, and diminished attention. But these associations are based on cross-sectional studies in which the temporal orders of cause and effect are unclear. 

Marijuana laws and increased use among adolescents

Marijuana use has been on the rise among US adolescents ever since the mid-2000s.

In a Lancet study, researchers mined 24 years of US data to assess the relationship between state marijuana laws and adolescent use of the drug. In a national, multistage, random sampling of 8th, 10th, and 12th graders, participants answered questions regarding marijuana use on an annual basis. Between 1991 and 2014, 1,098,270 students responded, with the primary outcome being any marijuana use during the past 30 days.

The investigators found that marijuana use was most frequent in states where medical marijuana laws had been passed (up to 2014). But the risk of marijuana use did not significantly differ before and after the laws were passed.

“Our findings, consistent with previous evidence, suggest that passage of state medical marijuana laws does not increase adolescent use of marijuana,” the authors concluded. “However, overall, adolescent use is higher in states that ever passed such a law than in other states. State-level risk factors other than medical marijuana laws could contribute to both marijuana use and the passage of medical marijuana laws, and such factors warrant investigation.”

Marijuana use and opioid overdose

In a separate Lancet study, Australian researchers followed a cohort of patients with chronic pain that wasn’t related to cancer. Patients were recruited from pharmacies across Australia and completed baseline interviews (n=1,514), as well as phone-interview follow-ups or self-completed questionnaires annually for 4 years. Participants answered questions regarding duration of non-cancer pain, chronic pain conditions, cannabis use, and more.

The researchers found that marijuana use was common among those samples, with 24% taking the drug. Furthermore, those who used the drug were in greater pain, and had more symptoms of generalized anxiety. However, no evidence of a temporal link between marijuana use and pain severity was exhibited. Importantly, cannabis use did not decrease prescribed opioid use or increase rates of opioid discontinuation.

“Cannabis use was common in people with chronic non-cancer pain who had been prescribed opioids, but we found no evidence that cannabis use improved patient outcomes,” the researchers wrote.

“People who used cannabis had greater pain and lower self-efficacy in managing pain, and there was no evidence that cannabis use reduced pain severity or interference or exerted an opioid-sparing effect. As cannabis use for medicinal purposes increases globally, it is important that large well designed clinical trials, which include people with complex comorbidities, are conducted to determine the efficacy of cannabis for chronic non-cancer pain,” they concluded.

Bottom line

Based on a status-quo understanding of a burgeoning marijuana landscape, it’s tempting to make certain assumptions. However, not all assumptions are borne out in the research.

It should be stressed, however, that each of these studies had certain limitations and further research needs to be done. For example, all these studies relied on self-reporting of marijuana use, which could be biased. Thus, these results, although informative, are still emergent.

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