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Studies Into Marijuana’s Mental Health Benefits Are Weak and Don’t Endorse Use, Review Finds

By Leo Bear-McGuinness

Published: Oct 28, 2019   
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There is barely any evidence to suggest that cannabinoids can treat depression, anxiety, and other mental health disorders, according to a new review of scientific studies.

Published in the Lancet Psychiatry, the “most comprehensive review of the evidence to date” found little evidence to support medicinal cannabis or pharmaceutical CBD’s use to relieve depression, anxiety, attention-deficit hyperactivity disorder (ADHD), Tourette syndrome, post-traumatic stress disorder (PTSD), or psychosis.

However, most of the 83 studies examined pharmaceutical grade THC or CBD rather than medical cannabis, so the review’s relevance to many medical marijuana patients may be tangible.

Cannabis use for mental health disorders

Second to chronic pain, mental health issues are one of the most common reasons patients seek out medical cannabis. In one patient survey from 2016, 36 percent of respondents said they used cannabis to treat a mental health condition – the most common being depression.

As cannabinoids can module the body’s natural endocannabinoid system, which has been recognized as an important factor in the body’s response to depressive and stress symptoms, some clinicians have speculated that the cannabis chemicals could form the basis of new therapeutics for mental health disorders.

Indeed, many US state departments include disorders such as Tourette Syndrome and PTSD as qualifying conditions for a medical marijuana prescription.

But, according to the new review, the science supporting these prescriptions is scarce.

“There is a notable absence of high-quality evidence to properly assess the effectiveness and safety of medicinal cannabinoids compared with placebo,” said Louisa Degenhardt, a professor at the University of New South Wales and lead author of the study, in a statement. “And until evidence from randomised controlled trials is available, clinical guidelines cannot be drawn up around their use in mental health disorders.”

Inside the review

Degenhardt’s research team reviewed 83 studies published and unpublished between 1980 and 2018, 40 of which were randomised controlled trials (RCTs) – the gold-standard test for new clinical drugs.

Among the results, they found that pharmaceutical THC–CBD didn’t significantly improve symptoms of depression, Tourette syndrome, or ADHD compared with a placebo. Indeed, instead of benefiting the patients, the cannabis compounds reportedly worsened the negative symptoms of psychosis.

And although some “very-low quality evidence” was found to support pharmaceutical THC’s use in anxiety treatments, these benefits were only seen in individuals with other medical conditions, such as multiple sclerosis.

This type of flaw was mirrored in many of the other studies reviewed. For example, not one of the participants in the THC-CBD trial for depression actually had a primary diagnosis of the disorder.

And as most of the studies examined pharmaceutical-grade cannabinoids rather than the plant material and extracts used by many medical cannabis patients, the review’s conclusions may not relate exactly to the experiences of many patients. None of the RCTs, for example, examined the effects of such medical cannabis on anxiety, ADHD, Tourette syndrome, PTSD, or psychosis.

Given this paucity of studies relevant to patients’ actual treatments, more research and RCTs will be needed before stronger conclusions can be made.

“Clinicians and consumers need to be aware of the low quality and quantity of evidence for the effectiveness of medicinal cannabinoids in treating mental health disorders and the potential risk of adverse events,” said Degenhardt. “Given the likely interest but scant evidence to guide patient and clinician decisions around cannabinoids for mental health, there is an urgent need for randomized controlled trials to inform whether there are benefits of cannabinoids for these indications.”

Leo Bear-McGuinness

Science Writer & Editor

Leo joined Analytical Cannabis in 2019. From research to regulations and analysis to agriculture, his writing covers all the need-to-know news for the cannabis industry. He holds a Bachelor's in Biology from Newcastle University and a Master's in Science Communication from the University of Edinburgh.


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