There's moderate- or high-quality evidence medical marijuana could help with some ailments, but the evidence is pretty weak for others, according to two reviews of studies published in the Journal of the American Medical Association.
The first study found moderate-quality evidence medical marijuana could be used to treat chronic pain and spasticity due to multiple sclerosis.
Researchers also found low-quality evidence the drug could work for nausea and vomiting that comes with chemotherapy, helping HIV patients gain weight and treating sleep disorders and Tourette syndrome, although many of the studies researchers reviewed didn't have results that were strong enough to show marijuana had a statistically significant effect that made it better than a placebo.
That review, which was conducted primarily by researchers in the Netherlands and United Kingdom, reviewed 79 randomized trials involving more than 6,400 participants conducted between 1975 and 2014 in a variety of countries.
The second study came back with similar findings. In that trial, researcher Kevin P. Hill with McLean Hospital and Harvard Medical School reviewed 28 randomized control trials, finding high-quality evidence marijuana could be used to treat chronic pain, neuropathic pain and spasticity related to multiple sclerosis.
The first study was funded by the Swiss Federal Office of Public Health. Hill, the author of the second study, disclosed he earns money from a book he wrote about marijuana, has received grants from the Brain and Behavior Research Foundation and the American Lung Association and gets his major funding from the National Institute on Drug Abuse.
Marijuana has drawbacks, too, researchers caution
The first study also found some people had short-term negative effects, which included things like dizziness, euphoria, nausea and dry mouth – and more rarely serious effects, including hallucinations and paranoia. Researchers couldn't find any studies looking at medical marijuana's long-term adverse effects.
When it comes to deciding whether to try medical marijuana, "individuals considering cannabinoids as a possible treatment for their symptoms should discuss the potential benefits and harms with their doctor," lead author of the first study Penny Whiting from the University of Bristol and Bristol NHS Foundation Trust said to Reuters in an email.
For instance, the second study said marijuana can be addictive and has been shown to cause "significant problems" for 9 percent of adult users and 17 percent of younger users. It can also make things like anxiety disorders worse, and is associated with anxiety, depression and psychotic illness in regular users.
Calls for more research
When it comes to definitively saying whether marijuana is useful for things like weight gain for HIV patients, anxiety disorders and assessing adverse effects, "[f]urther large, robust, [random control trials] are needed to confirm the effects of cannabinoids," authors of the first study wrote.
An accompanying editorial made that point even more strongly. Doctors Deepak Cyril D'Souza and Mohini Ranganathan, both with the Yale University School of Medicine, said the Food and Drug Administration requires evidence from at least two large random control trials before approving a drug.
But in the 23 states that have approved medical marijuana, "for most qualifying conditions, approval has relied on low-quality scientific evidence, anecdotal reports, individual testimonials, legislative initiatives, and public opinion. Imagine if other drugs were approved through a similar approach," they wrote.
"[I]f the states’ initiative to legalize medical marijuana is merely a veiled step toward allowing access to recreational marijuana, then the medical community should be left out of the process, and instead marijuana should be decriminalized," they continued. "Conversely, if the goal is to make marijuana available for medical purposes, then it is unclear why the approval process should be different from that used for other medications."