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As described in the journal JAMA, the team set out to review the benefits of cannabinoids in the treatment of various diseases or symptoms. Almost 80 randomized clinical trials were included in the study, which revealed that the majority found an improvement in symptoms with the use of cannabinoids. While this may sound positive, they actually found that most of these associated health improvements were not statistically significant.

Medical marijuana has not been proven to work for many illnesses that state laws have approved it for, according to the first comprehensive analysis of research on its potential benefits.

The strongest evidence is for chronic pain and for muscle stiffness in multiple sclerosis, according to the review, which evaluated 79 studies involving more than 6,000 patients. Evidence was weak for many other conditions, including anxiety, sleep disorders, and Tourette’s syndrome and the authors recommend more research.

The analysis is among several medical marijuana articles published Tuesday in the Journal of the American Medical Association. They include a small study suggesting that many brand labels for edible marijuana products list inaccurate amounts of active ingredients. More than half of brands tested had much lower amounts than labeled, meaning users might get no effect.
The analysis

The researchers pooled results from studies that tested marijuana against placebos, usual care or no treatment. That’s the most rigorous kind of research but many studies found no conclusive evidence of any benefit. Side effects were common and included dizziness, dry mouth and sleepiness. A less extensive research review in the journal found similar results.

It’s possible medical marijuana could have widespread benefits, but strong evidence from high-quality studies is lacking, authors of both articles say.

“It’s not a wonder drug but it certainly has some potential,” said Dr Robert Wolff, a co-author and researcher with Kleijnen Systematic Reviews Ltd, a research company in York, England.
Edible products

Researchers evaluated 47 brands of medical marijuana products, including candy, baked goods and drinks, bought at dispensaries in Los Angeles, San Francisco and Seattle.

Independent laboratory testing for THC, marijuana’s leading active ingredient, found accurate amounts listed on labels for just 13 of 75 products. Almost 1 in 4 had higher amounts than labeled, which could cause ill effects. Most had lower-than-listed amounts. There were similar findings for another active ingredient. Products were not identified by name.

Johns Hopkins University researcher Ryan Vandrey, the lead author, said he was surprised so many labels were inaccurate. The researchers note, however, that the results may not be the same in other locations.
Marijuana laws

Twenty-three US states and Washington DC have laws permitting medical marijuana use. Approved conditions vary but include Alzheimer’s disease, epilepsy, glaucoma, kidney disease, lupus and Parkinson’s disease.

An editorial in the journal says approval in many states has been based on poor quality studies, patients’ testimonials or other nonscientific evidence.

Marijuana is illegal under federal law, and some scientists say research has been stymied by government hurdles including a declaration that marijuana is a controlled substance with no accepted medical use.

But in a notice published Tuesday in the Federal Register, the Department of Health and Human Services made it a little easier for privately funded medical marijuana research to get approved. The department said that a federal Public Health Service review of research proposals is no longer necessary because it duplicates a required review by the Food and Drug Administration.
The future

Colorado, one of a few states where recreational marijuana use is legal, has pledged more than $8m in state funds for several studies on the drug’s potential medical benefits, including whether it can reduce veterans’ symptoms of post-traumatic stress disorder. That study may begin recruiting participants later this year, said Vandrey, one of that study’s leaders.

Vandrey said there’s a feeling of optimism in the research community that “we’ll start to get a good science base” for the potential medical uses of marijuana.
The recommendations

The editorial by two Yale University psychiatrists suggests enthusiasm for medical marijuana has outpaced rigorous research and says widespread use should wait for better evidence. Federal and state governments should support and encourage such research, the editorial says.

“Perhaps it is time to place the horse back in front of the cart,” Drs Deepak Cyril D’Souza and Mohini Ranganathan wrote in the editorial.

They note that repeated recreational marijuana use can be addictive and say unanswered questions include what are the long-term health effects of medical marijuana use and whether its use is justified in children whose developing brains may be more vulnerable to its effects.

Medical marijuana offers only weedy health benefits
Don't expect marijuana to perform medical miracles. That's the message from a huge review of 79 clinical trials undertaken between 1975 and 2015 that assessed the merit of taking cannabinoids – the active component in cannabis – to treat different health conditions.

Over the last 20 years, Washington DC and 23 US states have introduced laws to allow the use of medical marijuana to treat many conditions, including long-term pain, sleep problems and muscle disorders.

Penny Whiting of the University of Bristol, UK, and her colleagues set out to examine the evidence that the drug is actually beneficial, but found little to suggest it is.

"Most trials reported greater improvement in symptoms with cannabinoids compared to control groups," says Whiting – but they didn't usually reach statistical significance.

In addition, studies that explored the effects of medical marijuana were not always performed to a high standard. Many suffered from methodological weaknesses, such as a small sample size or incomplete data on outcomes, and substantial withdrawals of participants in half the trials. Any of these factors could have skewed the results.

Studies of a moderate standard supported the view that taking marijuana provides benefits for chronic pain relief and for muscle problems associated with conditions such as cerebral palsy. Studies of a lower standard have reported positive effects for treating sleep problems, Tourette's syndrome, sickness from chemotherapy and weight gain in people with HIV.

Medical marijuana is also taken by some people with conditions such as depression, although Whiting's review found little to no scientific evidence that this is beneficial. "We found no studies in patients with depression, and only one or two small studies in patients with anxiety disorder, psychosis or glaucoma," says Whiting. This doesn't mean that cannabinoids are ineffective for treating these conditions, she says – just that there is not yet enough evidence to reach any firm conclusions.
Trial bypass

Whiting says that cannabinoids have generally been approved for medical applications without having to go through the strict proof-of-benefit trials used to judge other medicines.

"I think cannabinoids should be evaluated in the same way as any other type of medical treatment," she says. "It's important that all interventions are judged by the same standards, so the potential benefits and adverse effects of cannabinoids should be considered in the light of the evidence."

The call for proper, thorough trials is echoed by Deepak Cyril D'Souza and Mohini Ranganathan at the Yale University School of Medicine in New Haven, Connecticut, in an accompanying commentary. They say there is a failure to test the drugs in the same way as others, through the US Food and Drug Administration, which is akin to "putting the cart before the horse".

"If a state's initiative to legalise medical marijuana is merely a veiled step towards allowing access to recreational marijuana, then the medical community should be left out of the process," says D'Souza. "Conversely, if the goal is to make marijuana available for medical purposes, then it's unclear why the approval process should be different from that used for other medications."

In a further blow to medical marijuana, a study in the same issue of JAMA found that edible cannabinoid pills often contained incorrectly labelled dosages. Of 75 products purchased, only 17 per cent were correctly labelled, with 60 per cent containing more than stated and 23 per cent less than stated of the active ingredient.

Journal reference: JAMA, DOI: 10.1001/jama.2015.6358