Many of the studies suggesting cannabinoids are effective for treating certain medical conditions are of low-moderate quality, according to the authors of the meta-analysis.
To date, 23 US states and Washington, DC, have legalized marijuana for medical use, with a further seven states pending legislation.
While the US Food and Drug Administration (FDA) have not approved the marijuana plant as a form of medicine, studies of the drug's active compounds, known as cannabinoids, have led to the approval of two medications - dronabinol and nabilone - that contain delta-9-tetrahydrocannabinol (THC), the primary cannabinoid in marijuana.
Both dronabinol and nabilone are used to treat nausea and vomiting caused by chemotherapy among individuals in whom other medications have failed, while dronabinol is also used to treat weight loss and loss of appetite for people with HIV/AIDS.
Previous studies have suggested that cannabinoids can also help treat chronic pain, Tourette syndrome, sleep disorders and other medical conditions.
However, the team involved in this latest study - including Penny F. Whiting, PhD, of the University of Bristol in the UK - notes that, despite marijuana being widely used for medical purposes in the US and some other countries, the effectiveness of the drug in treating certain medical conditions is unclear.
Low-, moderate-quality evidence supporting effectiveness of cannabinoids for medical use
With this in mind, Whiting and colleagues conducted a review of 79 randomized clinical trials involving 6,462 participants that assessed the effectiveness of cannabinoids in treating the symptoms of an array of medical conditions.
While the majority of studies indicated that cannabinoids could successfully treat symptoms of certain medical conditions, the researchers found many of these studies were not statistically significant.
When it came to the use of cannabinoids for treatment of weight loss in HIV/AIDS patients, nausea and vomiting due to chemotherapy, sleep disorders and Tourette syndrome, the researchers found there was low-quality evidence that the compounds were effective, while there was very low-quality evidence supporting the effectiveness of cannabinoids in the treatment of anxiety.
The studies that found no effect on psychosis with cannabinoid use were of low-quality, the researchers found, and there was very low-level evidence that cannabinoids were ineffective against depression.
Whiting and colleagues also assessed the occurrence of adverse side effects associated with cannabinoid use, which were reported in 62 of the studies included.
They found cannabinoid use increased the risk of short-term adverse side effects. Dry mouth, dizziness, fatigue, nausea, euphoria, vomiting, disorientation, confusion, loss of balance and hallucination were among the most common adverse events identified.
These findings did not differ depending on the type of cannabinoids used or the way they were administered, according to the researchers.
They note that only two of the studies assessed the effectiveness of cannabis for medical use, though they found no evidence that cannabis produced different results to other cannabinoids.
Based on their results, Whiting and colleagues say:
"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 adverse events of cannabis."
In an editorial linked to the study, Drs. Deepak Cyril and Mohini Ranganathan, of the Yale University School of Medicine in New Haven, CT, say if US states wish to make marijuana widely available for medical purposes, then more rigorous research into the medical benefits of the drug is required.
"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," they add. "Perhaps it is time to place the horse back in front of the cart."
In July 2014, a Spotlight feature from Medical News Today looked at the debate surrounding medical marijuana, which highlighted concerns from some physicians about the lack of reliable evidence supporting use of the drug for medical purposes.