The National Academy of Sciences has just completed one of the most comprehensive reviews ever done of marijuana research. In a report released yesterday, doctors and scientists from a wide range of disciplines evaluated more than 10,000 scientific abstracts from which they drew nearly 100 conclusions about the drug’s therapeutic value, its potential for abuse, and its link to diseases ranging from schizophrenia to cancer.

Their findings offer some validation to medical marijuana proponents who have long argued that the drug is effective against a range of maladies, from chronic pain to multiple sclerosis. But owing to a dearth of reliable, high-quality data and several contradictory conclusions, the report as a whole does little to resolve long-standing debates about the risks and benefits of cannabis use.  

Chronic Confusion

When it comes to medicine, few chemicals are as confusing as cannabis. On the one hand, the drug is hugely popular and increasingly legal: Twenty-eight states and the District of Columbia have legalized it for medical purposes; eight of those states plus D.C. have legalized it for recreational use. And according to a recent national survey, 22 million Americans over the age of 12 reported using cannabis in some form or other in the past 30 days.

But the plant is is still illegal at the federal level. The Drug Enforcement Agency (DEA) classifies it as a Schedule I substance, meaning that, in their estimation, it has no medical value and a high potential for abuse.

As a result of that classification, reliable scientific research into the plant’s risks and benefits has been paltry, and guidelines for safe and effective use of the drug (as a medication or for fun) remain all but nonexistent, even as its popularity continues to grow.

According to the new report, this failure to provide guidance qualifies as a significant public health threat. “Unlike alcohol and tobacco, which may cause harm, no accepted standards exist for marijuana to help guide individuals as they decide if, when, where, and how to use cannabis,” says Marie McCormick, M.D., a professor of pediatrics at Harvard Medical School and chair of the NAS committee that issued the report.

The report’s authors make several recommendations for repairing this deficit. 

More Research Needed

The report’s most significant conclusion is that we urgently need more and better research into the health effects of cannabis consumption.

The committee called on federal agencies — including the Centers for Disease Control and Prevention (CDC), Food and Drug Administration (FDA) and National Institutes of Health (NIH) — to develop a comprehensive national cannabis research agenda aimed at finding and closing key gaps in our current understanding of the drug’s risks and benefits. Those gaps include under-researched populations (such as pregnant women and people older than 50) and a lack of comparison between different doses and different modes of delivery for the treatment of any given ailment.

The report acknowledges that a primary reason for such gaps is the classification of cannabis as a Schedule I substance. But the authors stopped short of calling on the federal government to reclassify the drug, saying that such a recommendation would be beyond the scope of their mandate.

Instead, they urged those same federal agencies to take steps to expand access to research-grade marijuana. Right now, just one scientist at the University of Mississippi has license to grow and distribute cannabis to the research community, and scientists have long complained that this monopoly makes rigorous studies of the plant all but impossible.

For the current report, the authors were only able to map correlations between cannabis use and certain health outcomes; they were not able to say whether the drug actually caused those outcomes.

With that caveat in mind, here is a partial list of their findings:

The Available Evidence

The strongest evidence of benefit was for chronic pain and chemotherapy-induced nausea and vomiting. The committee found conclusive or substantial evidence of a link between the consumption of cannabis and the alleviation of both conditions.

By contrast, they found only limited evidence that cannabis use is linked to an improvement in the symptoms of Tourette’s syndrome, social anxiety, post-traumatic stress disorder (PTSD), or HIV-AIDS-related wasting.

And there was no or insufficient evidence tying cannabis products to an improvement in irritable bowel syndrome, epilepsy, spasticity related to spinal-cord injury, or motor symptoms related to Parkinson’s disease.

Accidents: The committee found substantial evidence of a link between cannabis use and increased risk of motor vehicle accidents; moderate evidence that the likelihood of overdose injuries in children increases when the drug is legalized; and no or insufficient evidence that marijuana use increases the risk of work-related accidents, overdose deaths, or death from any other cause.

Pregnant women: They found substantial evidence of a link between cannabis use and low birth weight, but only limited evidence that the drug is tied to a greater risk of pregnancy complications or NICU (neonatal intensive care unit) admissions.

Heart and lung conditions: There was only limited evidence of a connection between cannabis use and heart attacks or strokes.

There was substantial evidence that cannabis use is associated with respiratory symptoms, including a greater frequency of chronic bronchitis episodes. But the evidence of a link between smoking cannabis and developing COPD (cardio-obstructive pulmonary disorder) was limited. And there was no or insufficient evidence of a link between smoking cannabis and developing or exacerbating asthma.

Multiple sclerosis: Evidence of a link between cannabis use and an improvement in patient-reported symptoms of spasticity was also substantial. But when spasticity in MS patients was measured by clinicians, the link between cannabis use and relief was limited. The report did not speculate on the reasons for this discrepancy, or say whether the effects experienced by patients might be placebo-related. 

Cancer: There was no evidence that cannabis in any form can treat cancer, including brain and spinal tumors called gliomas. There was moderate evidence that the drug is not associated with lung, head and neck cancers (a surprise, given that all three have been linked to smoking tobacco). There was limited evidence of a link between frequent or chronic cannabis use and testicular tumors, but no evidence to say either way whether cannabis is linked to esophageal, prostate, cervical, or bladder cancer.

Schizophrenia (and other serious mental illnesses): The committee found substantial evidence of a link between cannabis use and the development of schizophrenia, though none of the studies they assessed took genetic factors into consideration or sought to determine which populations might be at greater risk of developing schizophrenia in conjunction with marijuana use. They found limited evidence of a link between marijuana use and an increase in schizophrenia symptoms in those already diagnosed with the disorder.

There was no evidence that cannabis use reduces hallucinations resulting from schizophrenia, but moderate evidence of improved cognitive performance in people with psychotic disorders (such as schizophrenia) who also have a history of cannabis use.

In people with bipolar disorder, the evidence suggests that regular cannabis use can be linked to a higher incidence of manic episodes, but so far there is only limited proof of a link between consuming cannabis and developing this disorder. 

Addiction: The authors found substantial evidence that the earlier someone starts using marijuana, the more likely they are to develop what the report authors call “problem use.” They also found moderate evidence of a link between cannabis use and substance dependence or abuse disorders. But evidence that cannabis use will lead a person to smoke or trigger a change in their rates or patterns of other drug use, legal or illegal, was limited.

Buyer Beware

Given the lack of solid evidence, consumers should play it safe when it comes to using cannabis products, either recreationally or as medicine. 

If you suffer from one of the conditions for which the drug has shown some evidence of a benefit (like multiple sclerosis, chronic pain, or chemotherapy-induced nausea) you should still talk to your doctor before trying cannabis-based products to treat your symptoms.

And you should be mindful of the fact that medical marijuana is a new and so far poorly regulated industry. Even products sold from dispensaries can have inaccurate labels or be contaminated with pesticides and other contaminants (and those chemicals can pose a particular risk to people with weakened immune systems). 

If you're using cannabis recreationally and are worried about drug dependence or other side effects, you should talk to your doctor about programs to help you quit. 

Last but not least, keep in mind that marijuana and other cannabis products are still illegal in many states.