Study of 10,000 reports into cannabis finds only enough evidence to support therapeutic use for chemotherapy patients, chronic pain and multiple sclerosis

There is not enough research to reach conclusive judgments on whether marijuana can effectively treat most of the symptoms and diseases it is advertised as helping, according to a wide-ranging US government study.

The same is also true of many of the risks said to be associated with using cannabis, the study finds.

More than 100 conclusions about the health effects of marijuana, including claims of both helpful and harmful effects, were evaluated by the National Academies of Sciences, Engineering and Medicine in a study released on Thursday.

There was only enough evidence to support treatment for three therapeutic uses, the study found: to reduce nausea and vomiting from chemotherapy, to treat chronic pain and to reduce spasms from multiple sclerosis.

“Really, most of the therapeutic reasons people use medical marijuana aren’t substantiated beneficial effects of the plant,” said Sean Hennessy, a professor of epidemiology at the University of Pennsylvania, and a member of the 16-scientist committee that carried out the review.

Uses for which there was either “limited evidence or insufficient evidence”, according to Hennessy, included increasing appetite and weight gain for patients with HIV/Aids, calming attention deficit hyperactivity disorder (ADHD) in children, and treating epilepsy.

“There’s been an explosion of literature since 1999 … We reviewed thousands of abstracts,” said Robert Wallace, a professor of epidemiology at the University of Iowa College of Public Health, and another member of the committee tasked with reviewing more than 10,000 studies to reach conclusions about the current state of research on marijuana. “A lot of the report is really where the state of the evidence is.”

For example, the report found “conclusive” evidence that cannabis can alleviate some nausea and vomiting associated with cancer treatment, but that case was long ago considered settled. For many indications in the report, science has not reached a definitive answer.

The report’s key findings fell into a few broad categories:

Injury and death : evidence suggests that driving while high increases the risk of a car accident. In states where marijuana has been legalized, evidence suggests that children are at more risk of ingesting marijuana. Cannabis use could also harm adolescents’ educational and social development.

: evidence suggests that driving while high increases the risk of a car accident. In states where marijuana has been legalized, evidence suggests that children are at more risk of ingesting marijuana. Cannabis use could also harm adolescents’ educational and social development. Mental health : cannabis use is likely to increase the risk of developing schizophrenia, social anxiety disorders and, to a lesser extent, depression. Heavy marijuana users are more likely to report suicidal thoughts that non-users, and people with bipolar disorder who use marijuana almost daily show more symptoms than non-users.

: cannabis use is likely to increase the risk of developing schizophrenia, social anxiety disorders and, to a lesser extent, depression. Heavy marijuana users are more likely to report suicidal thoughts that non-users, and people with bipolar disorder who use marijuana almost daily show more symptoms than non-users. Cancer : evidence does not support a link between smoking marijuana and cancers typically associated with tobacco, such as head, neck and lung cancer. Evidence suggests that smoking marijuana on a regular basis is associated with chronic bronchitis and phlegm production.

: evidence does not support a link between smoking marijuana and cancers typically associated with tobacco, such as head, neck and lung cancer. Evidence suggests that smoking marijuana on a regular basis is associated with chronic bronchitis and phlegm production. Addiction : evidence suggests that people who use more cannabis are more likely to be addicted, and that the younger people start, the more likely they are to develop problematic use.

: evidence suggests that people who use more cannabis are more likely to be addicted, and that the younger people start, the more likely they are to develop problematic use. Nausea : there is conclusive evidence that cannabinoids, compounds derived from marijuana, are useful to treat nausea and vomiting associated with chemotherapy.

: there is conclusive evidence that cannabinoids, compounds derived from marijuana, are useful to treat nausea and vomiting associated with chemotherapy. Chronic pain: there is evidence to support use of cannabis in treatment of chronic pain, in particular in spasms associated with multiple sclerosis, the disabling central nervous system disease.

The report comes as the pace of marijuana reforms has quickened across the US. Medicinal marijuana is now legal in 29 states, and recreational marijuana is legal in eight states and in Washington DC. Marijuana is the most widely used illicit drug in the US, with an estimated 22.2 million people using the drug in the past month, according to a 2014 government survey.

Drug policy reform has also became a topic of international debate, as high-profile medical organizations and some countries call for decriminalization of illicit drugs. The academies’ report is the first in 18 years to address such a breadth of physical effects from marijuana, and is likely to fuel arguments on both sides of the marijuana debate.



While the paper is broad and wide-ranging, it is unlikely to end debate on any number of treatments. One researcher studying the therapeutic impacts of the cannabis compound cannabidiol on children with epilepsy said researchers’ conclusions were broadly “conservative”, and on epilepsy “wrong”.



“For science to do what it does, I think when they do a scientific review they should incorporate all the evidence that is out there for that disorder, and I don’t think they did that for epilepsy,” said Orrin Devinsky, director of New York University Langone Medical Center’s comprehensive epilepsy center. Still, he called the report “very valuable”.



The report comes at a significant time for marijuana policy worldwide, as drug policy reform has picked up both in the US and internationally. Less than one year ago, an international commission published a report in The Lancet calling for decriminalization of all drugs, finding that prohibition did not effectively combat drug use, addiction or organized crime. At the same time, the United Nations held a special session to discuss global drug policy, the first in almost 20 years. Additionally, mental health concerns about “skunk” marijuana have also increased.

The study represents the broadest review by the National Academies of Sciences since 1999, after California and Arizona passed the first medical marijuana laws, and the public debated whether physicians should prescribe marijuana.

Controversially, the 1999 report found that marijuana could indeed reduce nausea, but that it could also help treat pain. While the report found some negative effects from smoking marijuana, it also contradicted the then US “drug czar” Barry R McCaffrey’s position that “not a shred of scientific evidence” existed for the medicinal use of marijuana.

Almost two decades later, legal marijuana has spawned an untold number of shops hawking marijuana for people to enjoy like beer or wine, but also products that more closely resemble cure-alls on the untested supplement market.

“There is a great fear that I continue to have, that the cannabis industry medically will be like the vitamin and nutritional supplement market,” Devinsky said. “People advertise on TV that we isolated a compound from jellyfish and it improves memory.”

He added: “It’s become a religion for people, and my personal view is if you want to pray to whatever gods you pray to, and you want to advocate for whatever political candidate, it’s a free country. But when it comes to medical therapy, we have a higher standard.”

In the US, the push for marijuana legalization comes at a time of increasing dismay over the state of the criminal justice system, as sentencing laws passed during the “war on drugs” are being reexamined in light of their disproportionate impact on people of color.

“This growing acceptance, accessibility and use of cannabis and its derivatives has raised important public health concerns,” said Marie McCormick, a pediatrician at Harvard University’s TH Chan School of Public Health, who chaired the committee. “Moreover, the lack of any aggregated knowledge of cannabis-related health effects has led to uncertainty about what, if any, are the harms or benefits from its use.

“We conducted an in-depth and broad review of the most recent research to establish firmly what the science says, and to highlight areas that still need further examination. As laws and policies continue to change, research must also.”

The report also took the unusual step of calling for private funding sources to advance marijuana research. Researchers found it “difficult to gain access to the quantity, quality, and type of cannabis product necessary”, the report said. “A diverse network of funders is needed to support cannabis and cannabinoid research.”

One of those hurdles is the limited supply of research-grade marijuana, which is grown only at the University of Mississippi.

“Any federal study has challenges,” said Nolan Kane, a geneticist at the University of Colorado at Boulder studying the evolution of plants such as sunflowers and hemp. He described government-approved marijuana as having “a fraction” of the psychoactive ingredients of privately grown marijuana, and said research subjects described the marijuana as “old” and “low quality”.

“Getting people to even accept money to smoke their marijuana – you’d think college campuses, it would be easy to get people to smoke marijuana,” Kane said. “It doesn’t at all reflect the marketplace ... It’s always going to be comparing apples and oranges until they’re able to make the products more similar.”

The report is the product of 16 experts in their fields, including neurologists, oncologists, epidemiologists and child psychiatrists. Among the review’s most fervent calls were for more investigation.

The study was sponsored by a group of state health departments, federal agencies and nonprofits, such as the Centers for Disease Control and Prevention, the National Highway Traffic Safety Administration, the Robert W Woodruff Foundation and the Arizona department of health services.