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Medical Benefits of Marijuana Unclear

Medical Benefits of Marijuana Unclear

Researchers at the University of Bristol, UK, have carried out an analysis of dozens of trials of the medical benefits of cannabis or marijuana.

Dr. Penny Whiting and her team looked at 79 randomized trials including 6,462 participants. Overall, this contained “moderate-quality evidence” to support the use of cannabinoids (chemical compounds that are the active principles in cannabis or marijuana) for chronic pain or spasticity due to multiple sclerosis.

But the evidence suggesting that cannabinoids were associated with improvements in nausea and vomiting due to chemotherapy, sleep disorders, and Tourette syndrome was “lower-quality,” they report. The evidence that cannabinoids could improve anxiety, depression, or psychosis was “very low-quality.”

The evidence for a beneficial effect on psychosis was “low-quality,” and there was “very low-level evidence” for an effect on depression. Neither the type of cannabinoids used, nor mode of administration, appeared to affect the results.

They explain in the Journal of the American Medical Association that most studies suggested cannabinoids were associated with improvements in symptoms, but these associations did not reach statistical significance in all studies.

Despite the introduction of laws to permit the medical use of cannabis in 23 states and Washington, D.C., “their efficacy for specific indications is not clear,” according to the team.

Short-term side effects of cannabinoids included dizziness, dry mouth, nausea, fatigue, somnolence, euphoria, vomiting, disorientation, drowsiness, confusion, loss of balance, and hallucination.

The authors write, “Further large, robust, randomized clinical trials are needed to confirm the effects of cannabinoids, particularly on weight gain in patients with HIV/AIDS, depression, sleep disorders, anxiety disorders, psychosis, glaucoma, and Tourette syndrome are required.

“Further studies evaluating cannabis itself are also required because there is very little evidence on the effects and side effects of cannabis,” the authors write.

Deepak Cyril D’Souza, M.B.B.S., M.D. of the Yale University School of Medicine, New Haven, Connecticut, commented on the findings in an editorial.

He writes, “There is some evidence to support the use of marijuana for nausea and vomiting related to chemotherapy, specific pain syndromes, and spasticity from multiple sclerosis. However, for most other indications that qualify by state law for use of medical marijuana, such as hepatitis C, Crohn disease, Parkinson disease, or Tourette syndrome, the evidence supporting its use is of poor quality.”

He points out, “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 … For most of the conditions that qualify for medical marijuana use, the evidence fails to meet FDA standards.

“If 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.

“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. Evidence justifying marijuana use for various medical conditions will require the conduct of adequately powered, double-blind, randomized, placebo/active controlled clinical trials to test its short- and long-term efficacy and safety. The federal government and states should support medical marijuana research.

“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 to guide the development of a rational approval process.”

In his work as a psychiatrist, D’Souza has extensively studied the impact of marijuana on mental health. He is concerned about how routine daily use may affect the body and the brain over the long term.

Dr. Suzi Gage, also from the University of Bristol, studied this issue and concludes, “Overall, evidence from epidemiologic studies provides strong enough evidence to warrant a public health message that cannabinoids can increase the risk of psychotic disorders.

“However, further studies are required to determine the magnitude of this effect, to determine the effect of different strains on risk, and to identify high risk groups particularly susceptible to the risk of psychosis.”

D’Souza adds, “We don’t fully understand why some people appear to be more vulnerable to these effects, but that is a devastating mental disorder for anyone to have.”

He agrees that cannabinoids are difficult to study because there are hundreds of different components in different strains, and he calls on federal and state health officials to remove any legal or financial obstacles to further investigation.


Whiting, P. F. et al. Cannabinoids for Medical Use: A Systematic Review and Meta-Analysis. The Journal of the American Medical Association, 24 June 2015 doi:10.1001/jama.2015.6358

Gage, S. H. et al. Association Between Cannabis and Psychosis: Epidemiologic Evidence. Biological Psychiatry, 12 August 2015 doi: 10.1016/j.biopsych.2015.08.001

Medical marijuana photo by shutterstock.

Medical Benefits of Marijuana Unclear

Jane Collingwood

Jane Collingwood is a long-time writer for Psych Central, with a background in journalism and a focus on mental health.

APA Reference
Collingwood, J. (2018). Medical Benefits of Marijuana Unclear. Psych Central. Retrieved on November 30, 2020, from
Scientifically Reviewed
Last updated: 8 Aug 2018 (Originally: 26 Sep 2015)
Last reviewed: By a member of our scientific advisory board on 8 Aug 2018
Published on Psych All rights reserved.