January 17, 2019

What we know (and don't) about marijuana's health risks

Daily Briefing

    The United States "is in the midst of a sea change" in marijuana-related policies, but when evaluating the potential health effects, Americans need to "use the best methods and evidence as a starting point," Aaron Carroll writes for the New York Times' "The Upshot."

    Carroll, a professor of pediatrics at Indiana University School of Medicine, rounds up the most relevant research available to date on the topic.

    What providers should know about the medical marijuana legal landscape

    Does marijuana legalization mean more car crashes?

    According to recent report from the National Security Council, car crashes are one of the top causes of accidental death in the United States—and some experts have questioned whether legalizing marijuana would lead to an increase in car accidents. However, according to Carroll, common conceptions about how drivers are tested for driving under the influence can lead to "misperception[s]" about how marijuana relates to car crashes.

    Carroll explains that when someone drives drunk, a breathalyzer test can detect whether the individual is "measurably impaired when [his or her] blood alcohol level is above a certain level." However, according to Carroll, "[t]he tests we use for measuring the presence of THC ... do not measure the level of impairment" but instead "measure whether someone has used marijuana recently." As a result, if marijuana were to be legalized, and more people were to use the drug, "more people will register its recent use," Carroll writes—even if those people aren't impaired.

    Further, Benjamin Hansen—a professor of economics at the University of Oregon who has studied homicide levels in Colorado and Washington before and after each state legalized marijuana—found that "marijuana-related fatality rates did not increase more after legalization than what you would expect from trends and other states," Carroll writes.

    Can it cause schizophrenia?

    Another question—and misconception—that arises around marijuana legalization is its potential relation to schizophrenia, Carroll writes, citing misinterpretations of the National Academies of Sciences, Engineering, and Medicine's report on marijuana.

    Ziva Cooper, an author of the report, said some people incorrectly concluded from the report's findings that cannabis causes schizophrenia. According to Cooper, the relationship between marijuana and schizophrenia was "stated as an association, not causation"—and the direction of that association is still unclear, Cooper said. In fact, Cooper said, when it comes to psychiatric disorders, using marijuana might have its benefits.

    That said, Cooper noted people with certain psychiatric disorders should proceed with caution, given that research showing a "strong association between cannabis use and schizophrenia means that people with predisposing risk factors for schizophrenia should most certainly abstain from using cannabis."

    Does marijuana influence crime rates?

    Carroll notes that some states, such as Colorado and Washington, have experienced an increase in crime since legalizing marijuana. But, according to Carroll, "it's ... reasonable to be skeptical about [the] causation."

    The best way to investigate the relationship between marijuana and the increase in crime rates, Carroll writes, would be through the synthetic control method. With this method, researchers would "use a weighted combination of similar groups (states that are like Colorado and Washington in a number of ways) to create a model of how … states might...perform with respect to crime," depending on marijuana's legality, Carroll writes.

    Carroll cites Hansen's research, which relied on a synthetic control method to create a comparison group with homicide levels similar to Colorado and Washington between 2000 and 2012—the year that the states legalized marijuana. Hansen's model showed that researchers "might have predicted more of an increase in Colorado or Washington just based on trends seen in comparable states, even without legalization," Carroll writes. In fact, the method showed that Colorado and Washington actually had lower crime rates after marijuana was legalized than what was expected given trends in similar states

    While the results are "not evidence that legalization lowers crime rates ... we shouldn't conclude that it increases them," Carroll writes.

    Replacing fear with evidence

    When it comes to evaluating the risks and benefits of marijuana, "we should be honest about what we do and don't know," Carroll writes. On the one hand, "no one should be under the impression that marijuana is harmless," Carroll states. At the same time, Carroll writes, "Nor should anyone be irrationally exuberant about its upsides."

    As marijuana becomes legal in more states, it "will affect more people," which makes it perfectly reasonable to feel "concerned" about the potential risks that might remain unknown, Carroll writes, adding that in fact, "experts...are still nervous about how we might proceed." To ensure that our understanding of marijuana is increasing as the laws continue to change, "we need more research" on the subject, Carrol writes (Carroll, "The Upshot," New York Times, 1/14).

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