Medical marijuana may provide some benefit for patients with chronic nerve pain or cancer pain, as well as people who have multiple sclerosis and experience muscle spasms, according to a new review study.

However, there is not much evidence supporting the use of medical marijuana for other reasons, such as sleep disorders, Tourette syndrome and anxiety disorders. And marijuana doesn't appear to help people with depression or psychosis, or those with eye pressure from glaucoma, the study found.

Still, many of the studies done to date that found that marijuana had little or no effect were small, or lacked a rigorous design, the researchers said. Larger, more robust studies are needed to confirm the medical effects of cannabinoids, the compounds that are the active ingredients in marijuana, the researchers said in their findings, published today (June 23) in the Journal of the American Medical Association.

"There is evidence to support the use of cannabinoids for the treatment of chronic pain and spasticity," said study co-author Penny Whiting, a senior research fellow in epidemiology and health services research at the University of Bristol in the United Kingdom "However, this needs to be balanced against an increased risk of side effects," which can include dizziness, dry mouth, nausea and sleepiness, Whiting said.

"Individuals considering cannabinoids as a possible treatment for their symptoms should discuss the potential benefits and harms with their doctor," Whiting said.

The new findings raise important issues, given that 23 states have laws allowing the use of medical marijuana for various conditions, said Dr. Deepak Cyril D'Souza and Dr. Mohini Ranganathan, of the Yale University School of Medicine, who wrote an editorial accompanying the new study.

"For most qualifying conditions [for medical marijuana], approval has relied on low-quality scientific evidence, anecdotal reports, individual testimonials, legislative initiatives and public opinion," D'Souza and Ranganathan wrote. "Imagine if other drugs were approved through a similar approach."

If the goal of legalizing medical marjiuana is to make it available for medical purposes, then "it is unclear why the approval process should be different from that used for other medications," D'Souza and Ranganathan said. "Since medical marijuana is not a life-saving intervention, it may be prudent to wait before widely adopting its use until high-quality evidence is available." [Where Americans Smoke and Grow Marijuana (Maps)]

The new review, analyzed information from nearly 80 studies that included about 6,500 people total, in which participants were randomly assigned to take cannabinoids, a placebo or another drug. The cannabinoids tested in the study included compounds from the cannabis plant, such as THC (tetrahydrocannabinol) and CBD (cannabidiol), or synthetic compounds such as dronabinol and nabilone.

According to an analysis of eight studies that looked at cannabinoids for patients with nerve and cancer pain, patients using the substances were about 40 percent more likely than the people using a placebo to report a reduction in their pain of at least 30 percent.

Some studies suggested that people with MS who took cannabinoids experienced improvements in their muscle spasms.

There was also some evidence that cannabinoids helped with nausea and vomiting in patients undergoing chemotherapy: In an analysis of three studies, 47 percent of patients using cannabinoids had their nausea and vomiting completely stop, while only about 20 percent in the placebo group had these symptoms completely stop. Some drugs, including dronabinol and nabiximol, are approved to treat chemotherapy-induced nausea and vomiting.

There was no evidence of benefit for people with depression, psychosis or glaucoma, but the studies of marijuana use by people with these conditions were small, and in the case of depression, the studies were conducted primarily to look at other outcomes, such as chronic pain.

Common side effects from marijuana included dizziness, dry mouth, nausea, fatigue, drowsiness, vomiting, disorientation, confusion, loss of balance and hallucination.

Although the acute effects of marijuana are well known, more studies are needed on the effects of repeated exposure to marijuana, which would occur among patients who use the drug regularly for medical purposes, the editorial said.

About 1 in 10 marijuana users develops addiction to the drug, and users can also develop a tolerance, meaning that the dosing of the drug may have to be increased over time to have the same effect, the editorial said.

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