Medical marijuana: good evidence for some diseases, weak for others

Moderate- or high-quality evidence supports the use of marijuana for some medical conditions, but not for others, according to a fresh review of past research.

After reviewing 80 randomized trials that included nearly 6,500 people, researchers found moderate support for using marijuana to treat chronic pain and muscle spasms and involuntary movements.

The evidence wasn't as strong to support marijuana's use for nausea and vomiting due to chemotherapy, sleep disorders, HIV-related weight loss and Tourette syndrome.

Also, any benefits of marijuana or cannabis use must be weighed against the risk of side effects, which include dizziness, dry mouth, nausea, sleepiness and euphoria, according to the study's lead author.

"Individuals considering cannabinoids as a possible treatment for their symptoms should discuss the potential benefits and harms with their doctor," said Penny Whiting of University Hospitals Bristol NHS Foundation Trust in the UK.

She also told Reuters Health by email that other reviews of medical marijuana suggest prolonged use may be tied to an increased risk of psychosis.

The new review, which is published in the Journal of the American Medical Association (JAMA), was commissioned by the Swiss Federal Office of Public Health. The researchers searched medical databases for past randomized controlled trials, which are considered the "gold standard" of medical research.

While the researchers found that most trials suggested some improvements in symptoms for the various conditions, not all could suggest the improvement wasn't just due to chance.

"As systematic reviewers, we have provided a summary of the available evidence which doctors can now use to make decisions regarding whether to prescribe cannabinoids for their patients," Whiting said.

A second review published in the same journal by Dr. Kevin Hill of McLean Hospital in Belmont, Massachusetts, found similar results.

In that review, Hill found high-quality evidence to support the use of marijuana in people with chronic or neuropathic (nerve) pain, and muscle problems related to multiple sclerosis.

"The two reviews have reached similar conclusions, that while there is some evidence to support the use of marijuana for certain conditions . . . for many of the other conditions that various U.S. states have approved medical marijuana, the evidence is of low quality," said Dr. Deepak Cyril D’Souza of Yale University School of Medicine in New Haven, Connecticut.

As of March 2015, 23 states and the District of Columbia allow marijuana use for medical purposes.

"If the primary process by which medications are approved for 'medical' use in the U.S. is the (Food and Drug Administration) approval process, then the evidence for many conditions does not meet the existing threshold of evidence," said D'Souza, who co-authored an editorial accompanying the new reviews.