By Andrew M. Seaman

(Reuters Health) - 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 published June 23 in JAMA was commissioned by the Swiss Federal Office of Public Health. The researchers searched medical databases for past randomized controlled trials.

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 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, Conn.

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.

In another study in the same journal, researchers found poor labeling on medical marijuana.

Of 75 edible marijuana products purchased in three metropolitan U.S. areas, less than one in five were labeled correctly, according to Ryan Vandrey of Johns Hopkins University in Baltimore and colleagues.

They found the vast majority of products contained more or less of the active ingredients than the label indicated.

In his editorial, D'Souza and his co-author say patients may have to experiment with different types of marijuana to achieve the desired effects.

"Both patients and doctors should demand that the government support high quality research to conclusively determine whether marijuana has therapeutic effects in the various conditions for which it has been currently approved," D'Souza said.

SOURCE: http://bit.ly/1TLbzaa, http://bit.ly/1IwCptX, http://bit.ly/1de01L8 and http://bit.ly/1Jgqnuf

JAMA 2015.