Rolling up (Image: ddp USA/REX)

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