Medical marijuana for mental illness? Scientists say not enough evidence yet


A new study has concluded that there is insufficient evidence to support claims that medicinal cannabinoids are a "cure-all" for people with mental illness, or, in fact, that it is able to improve mental health at all.

These findings, published in The Lancet Psychiatry journal, represent the most comprehensive analysis of medicinal cannabinoids and their impact on six mental health disorders and their symptoms, i.e. depression, anxiety, ADHD, Tourette syndrome, PTSD and psychosis.

The meta-analysis was based on a combination of 83 studies that included 3 000 people, and indicates that the use of cannabinoids for mental health conditions cannot be justified. This conclusion is based on the current lack of evidence for the effectiveness of these substances, as well as their known risks. 

Dangers of non-medicinal cannabis

Medical marijuana is fast becoming a go-to treatment for many people who suffer from chronic pain, and for this reason, is legally available in several countries, including South Africa.

Medicinal cannabinoids like tetrahydrocannabinol (THC) and cannabidiol (CBD), are increasingly being used around the world to treat mental health disorders.

However, there is a large body of evidence indicating that non-medicinal cannabis use can actually increase the occurrence of depression, anxiety and psychotic symptoms.

Professor Louisa Degenhardt of the National Drug and Alcohol Research Centre (NDARC) at UNSW Sydney, Australia, and lead author of the study said that the study’s findings have important implications in countries where cannabis and cannabinoids are being made available for medical use.

"There is a notable absence of high-quality evidence to properly assess the effectiveness and safety of medicinal cannabinoids compared with placebo, and until evidence from randomised controlled trials is available, clinical guidelines cannot be drawn up around their use in mental health disorders." 

In countries where medicinal cannabinoids are already legal, Degenhardt cautions that doctors and patients must be aware of the lack of evidence, as well as the risks of cannabinoids, and that these must be weighed when considering using it to treat symptoms of these common mental health disorders.

How the study was done

To examine the available evidence for all types of medical cannabinoids, the researchers studied and investigated 83 published and unpublished studies between 1980 and 2018. The sample included both randomised controlled trials (RCT) and open-label trials (participants know which treatment they’re taking). Of the studies:

  • 42 looked at depression
  • 31 looked at anxiety
  • 8 looked at Tourette syndrome
  • 3 looked at ADHD
  • 12 looked at PTSD
  • 11 looked at psychosis

In most RCTs examining depression and anxiety, the primary reason for cannabinoid use was for reasons unrelated to mental health, such as chronic non-cancer pain or multiple sclerosis

Few randomised controlled trials examined the role of pharmaceutical CBD or medicinal cannabis. Instead, most looked at THC, with or without CBD.

What they found


The researchers found "very low quality evidence" that the cannabinoid THC (with or without CBD) can improve anxiety symptoms in cases of chronic pain.

It is important to note, however, that this may have been due to improvements in patients' primary medical condition, the authors explain. For this reason, the authors suggest further research should be done to explicitly study the effects of cannabinoids on anxiety and depression.


Pharmaceutical THC (with or without CBD) worsened negative symptoms of psychosis and didn’t significantly affect any other primary outcomes for the mental health disorders examined.

The authors highlight the need for high-quality research as there is currently only limited and low quality evidence for the effectiveness of cannabinoids in the treatment of mental health conditions.

Downfall of this study

Since there is only a small amount of data and small study sizes available for analysis, any conclusions would be of limited value. 

The authors therefore conclude that despite cannabinoids hypothetically showing potential for a variety of health conditions, they shouldn't be promoted as a treatment for mental health until there is solid proof of their efficacy. 

Image: iStock

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