Yet for all its ubiquity, a comprehensive new report says the precise health effects of marijuana on those who use it remain something of a mystery — and the federal government continues to erect major barriers to research that would provide much-needed answers.
If historical patterns are any guide, ballot initiatives that legalized recreational marijuana in California, Maine, Massachusetts and Nevada last year will lead to an increase in cannabis use and drive down public perceptions of the drug's risks. The result could be a natural experiment on a grand scale, according to the report released Thursday by the National Academies of Sciences, Engineering and Medicine.
"This lack of evidence-based information on the health effects of cannabis and cannabinoids poses a public health risk," a panel of 16 experts concluded in the first comprehensive look at marijuana research since 1999.
The report, nine months in the making, assessed more than 10,000 studies that examined marijuana's relationship with cancer, psychiatry, accidents and a host of other health issues. The authors included physicians, public health experts, neurobiologists and addiction specialists.
California's Department of Public Health was one of 15 sponsors of the report. Department spokesman Matt Conens said in a statement that it hoped "to gather credible information" to protect patients and the public and guide the state's cannabis-related public health response, and is reviewing its recommendations.
Some things were clear. The report authors concluded with confidence that marijuana and products that mimic its psychoactive effects can provide effective treatment of chronic pain and help some patients with sleep. Cannabis and cannabinoids, they wrote, effectively ease chemotherapy-induced nausea in cancer patients and spasticity in those with multiple sclerosis.
There is "substantial evidence" that women who use marijuana during pregnancy are more likely to give birth to smaller babies who face a range of early disadvantages. The report also detailed strong evidence that long-term pot-smoking is linked to worsened respiratory symptoms and more frequent episodes of chronic bronchitis. And it found solid research findings of an overlap between frequent users of marijuana and those who develop schizophrenia or other psychotic disorders, though it's not clear whether one causes the other.
At the same time, the authors warned that many of the conditions for which patients have turned to medical marijuana have little or no research that demonstrates its effectiveness. Those include the use of cannabis or cannabinoids for treatment of epilepsy, Parkinson's disease symptoms and support of abstinence from addictive substances.
Also thin, according to the new report: research that clarifies the relationship between drugged driving and accidents. The statistical link between marijuana use and an increased risk of motor vehicle crashes is "substantial." But that might be because those most likely to drive under the marijuana's influence — young men and people who also use alcohol and other drugs — are already more likely to get in accidents.
While it's possible to measure the concentration of marijuana's active agent, THC, in the blood of drivers, researchers aren't really sure at what concentration impairment — or, for that matter, beneficial effects — kicks in, the experts wrote.
That's a crucial research gap for state legislators looking to draft laws against driving under the influence. Indeed, the panel members wrote, it's not even clear that measures focusing on marijuana use alone would save lives.
Overall, the report suggests that, like many drugs, marijuana can be powerful medicine at some doses and to some people, and potentially dangerous in other strengths and to other people. The body of available research doesn't provide a clear guide to who will reap those benefits or incur those harms, and how dosage or mode of administration could spell the difference.
"What do we really know for sure? Mainly it's anecdotes or very poor evidence," said Dr. Marie McCormick, a maternal and child health expert at Harvard's School of Public Health who chaired the National Academies panel.
Given the continuing tug-of-war between the states and the federal government over marijuana policy, it's unlikely that new research will provide better answers any time soon. Those who wish to investigate marijuana's effects face high legal and bureaucratic hurdles, the panel noted.
When most federally funded researchers put marijuana's properties to a rigorous test, they have one legal source to turn to: a University of Mississippi facility that has cultivated the plant for the National Institute on Drug Abuse since 1968.
Commercially available marijuana and derivative products have changed dramatically since then, becoming more potent, more concentrated and available in forms that can be consumed and vaped as well as smoked. These changes aren't reflected at the growing facility; in most cases, it offers the plant's leaves and flowers in a narrow range of concentrations. Only in July did NIDA ask researchers how its marijuana products might better serve their research needs, the panel said.
Rules governing marijuana research can also be forbidding, the panel added.
Federally funded marijuana researchers must get an OK from the Drug Enforcement Agency and in some states, a state board of medical examiners. The DEA requires researchers to erect elaborate security measures to limit the number of people who come into contact with marijuana provided for research.
"This process can be a daunting experience for researchers," the panel wrote.
Meanwhile, testing the health effects of marijuana products that are actually marketed to consumers is illegal, according to the report. Even as sales of cannabis concentrate (often called "dabs") doubled in Colorado between 2015 and 2016, federal law prevented biomedical researchers from conducting research on its benefits or harms, blocked chemists from examining its safety, and barred neuroscientists from gauging its effects on the brains even of lab animals.
While marijuana "edibles" are a booming part of the consumer market, federal law also bars scientists from testing these products for contaminants, investigating their effects on patients with certain medical conditions, or administering them to lab animals.
"The federal government continues to enforce restrictive policies and regulations on research into the health effects or harms of cannabis products that are available to consumers in a majority of states," the report said. Those strictures are "leaving patients, health care professionals and policy makers without the evidence they need to make sound decisions regarding the use of cannabis and cannabinoids."
Tarek Tabsh, who operates medical marijuana dispensaries in Los Angeles and Las Vegas, said he's concerned about the dearth of reliable information about the products his industry sells. In particular, he said, it's important to understand how the industry's customers will be affected by the increasingly concentrated products that are coming on the market.
"I question the value of a lot of current research," said Tabsh, who hailed the report's call for changes.
"The biggest fear I have has nothing to do with policy or commerce," he added. "It has everything to do with science."
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