What We Don't Know About Marijuana's Risks and Benefits

It fills a new book from the National Academy of Sciences.



The last time the National Academy of Sciences issued a report on marijuana, three states allowed medical use of the drug. Eighteen years later, there are 28 states that recognize marijuana as a medicine, and eight of them also allow recreational use. But as a new NAS report published yesterday shows, there are still big gaps in our knowledge of marijuana's risks and benefits.

The 1999 report, commissioned by a drug czar who insisted there was no evidence that marijuana is medically useful, refuted that claim but highlighted the paucity of relevant research. "The accumulated data indicate a potential therapeutic value for cannabinoid drugs, particularly for symptoms such as pain relief, control of nausea and vomiting, and appetite stimulation," it concluded.

The new report, which takes into account studies conducted during the last two decades, is less tentative but still finds the evidence for most medical applications inconclusive. "We found conclusive or substantial evidence…for benefit from cannabis or cannabinoids for chronic pain, chemotherapy-induced nausea and vomiting, and patient-reported symptoms of spasticity associated with multiple sclerosis," the authors say. "For these conditions the effects of cannabinoids are modest; for all other conditions evaluated there is inadequate information to assess their effects."

The report notes that investigation of marijuana's medical utility has been constrained by legal and bureaucratic barriers, including continued federal prohibition and the Drug Enforcement Administration's refusal to license more than one producer of cannabis for research. "There are specific regulatory barriers, including the classification of cannabis as a Schedule I substance, that impede the advancement of cannabis and cannabinoid research," the authors say. "It is often difficult for researchers to gain access to the quantity, quality, and type of cannabis product necessary to address specific research questions." Last August the DEA once again refused to reclassify marijuana but agreed to start accepting applications from additional marijuana producers.

As state-legal marijuana products proliferate across the country, federal prohibition prevents scientists from investigating their properties:

Cannabis concentrate sales doubled in Colorado from 2015 to 2016, reaching $60.5 million in the first quarter of 2016, and yet current federal law prevents chemists from examining the composition of those products as it may relate to safety, neuroscientists from testing the effects of those products on the brain or physiology in animal models, and clinical scientists from conducting research on how these products may help or harm patients. And while between 498,170 and 721,599 units of medical and recreational cannabis edibles were sold per month in Colorado in 2015, federal law also prohibits scientists from testing those products for contaminants, understanding the effects of these products in animal models, or investigating the effects in patient populations.

Regarding the potential dangers of these products, the report is mostly reassuring, finding little or no evidence that marijuana impairs the immune system or increases the risk of heart attacks, lung cancer, or chronic obstructive pulmonary disease (contrary to the claims of anti-pot activists). Regular pot smoking seems to worsen bronchitis symptoms, and marijuana consumption by pregnant women is associated with lower birth weight, although there is little evidence of a link to pregnancy complications or postnatal health problems.

Marijuana use is associated with schizophrenia, suicide, poor academic performance, and abuse of other drugs, but the causal relationships remain murky. The report says "there is limited evidence of a statistical association between sustained abstinence from cannabis use and impairments in the cognitive domains of learning, memory, and attention"—i.e., effects that persist long after people have stopped using marijuana, which remains a subject of much controversy.

The report notes that marijuana legalization has been followed by increases in accidental ingestion of cannabis by children, which is usually not medically serious but can depress respiration. Another concern about legalization is its impact on road safety, since "there is substantial evidence of a statistical association between cannabis use and increased risk of motor vehicle crashes."

Laboratory tests indicate that marijuana impairs driving ability, although not as dramatically as alcohol does. Measuring marijuana's impact on actual car crashes is difficult, however, not least because studies generally equate a positive test for marijuana with intoxication, treating sober drivers as if they were stoned. The report notes that a 2016 review of the research suggests marijuana has a "low to moderate" effect on crash risk, increasing it by 20 or 30 percent (compared to an increase of about 400 percent for drivers with a blood alcohol concentration of 0.10 percent). But it's not clear how many of the drivers who were deemed to be under the influence of marijuana in the studies covered by that review had consumed it recently enough that they were still feeling the effects.

For the most part, this report seems like an honest and careful attempt to grapple with what we know and don't know about marijuana, which means neither prohibitionists nor cannabis enthusiasts will be completely happy with it (although both will mine it for ammunition). The authors are generally careful about drawing causal conclusions from statistical associations.

But not always. The report says "cannabis use is likely to increase the risk for developing substance dependence" involving other drugs, a quote we surely will be seeing in anti-pot propaganda hyping marijuana's "gateway effect." Later in the same chapter, however, the authors concede that all we really know is that "there is moderate evidence of a statistical association" between cannabis consumption and abuse of other drugs. That's why "additional studies are needed to determine whether cannabis use is an independent risk factor for, or causally contributes to, the initiation or use of and dependence on other drugs of abuse later in life."

While I welcome the self-correction, I doubt research will ever resolve this issue. Scientists can (if regulators let them) conduct a randomized, double-blind study to see whether cannabis extract is effective at controlling seizures or treating irritable bowel syndrome. They cannot legally or ethically conduct such a study to see whether smoking pot makes teenagers more likely to become heroin addicts.

As researchers like to say, more research is needed, especially in an area where the government has systematically discouraged it. But research cannot solve every puzzle.