Mental health

No evidence that medical cannabis helps mental health conditions

"Risks of cannabis use for mental health treatment outweigh benefits", The Guardian reports, following the publication of a study on the treatment of mental health conditions using medical cannabis.

Medical cannabis is being increasingly used to treat various conditions, including chronic pain, epilepsy and certain mental health conditions such as depression and anxiety. There has been considerable media coverage in recent years reporting people's difficulty in accessing these medicines.

Now a large-scale review has looked at the evidence for the use of medical cannabis to treat mental health conditions. Researchers said most of the 83 studies they reviewed were small and of poor quality.

They found no evidence that medical cannabis improved depression, psychosis, attention deficit hyperactivity disorder (ADHD), post-traumatic stress disorder (PTSD) or Tourette's syndrome. Some trials found a small improvement in anxiety symptoms, but the quality of the evidence was very low.

The use of medical cannabis was also associated with side effects, including worsening of mental health symptoms.

There is currently not enough evidence to support the use of medical cannabis to treat mental health conditions. More high-quality research is needed.

We do know that regular use of "street cannabis" – especially the very strong skunk kind of cannabis – can increases your risk of developing a psychotic illness, such as schizophrenia. Find out more facts about cannabis.

Where did the story come from?

This study was conducted by researchers from the University of New South Wales and University of Queensland in Australia, and the National Addiction Centre at Kings College London.

Funding was provided by the Therapeutic Goods Administration and Commonwealth Department of Health in Australia, the Australian National Health and Medical Research Council, and the US National Institutes of Health.

The study was published in the peer-review medical journal The Lancet.

The UK media reporting of the study was accurate.

What kind of research was this?

This was a systematic review that searched the global literature to identify studies looking at the effect of medical cannabis on mental health conditions. The individual study results were then pooled in a meta-analysis.

Medical (or medicinal) cannabis is a broad term meaning a medicine that includes cannabinoid chemicals – natural extracts from the cannabis plant and man-made varieties. The 2 main types are tetrahydrocannabinol (THC) and cannabidiol (CBD).

A review and meta-analysis are the best way to gather evidence on the safety and effectiveness of a particular treatment. However, the strength and quality of evidence from a systematic review is only as good as the studies used.

Randomised controlled trials (RCT) are the best individual study design to examine the effect of a treatment as they should balance other differences between participants that could influence the results. This review identified a mix of trial models and observational studies.

What did the research involve?

The study authors searched online literature databases to identify studies published between 1980 and 2018 that examined the effect of medical cannabis on mental health conditions in adults. To be eligible for selection, the studies had to look at, among other things, whether medical cannabis led to an improvement of the condition (remission) or a change in symptoms.

The researchers identified a total of 83 studies, including 40 randomised controlled trials (RCT) including 3,067 adults, with the remainder being observational studies. These covered mental health conditions including:

  • depression: 23 RCTs including 2,551 adults (total 42 studies, including observational)
  • anxiety: 17 RCTs including 605 adults (total 31 studies)
  • psychosis: 6 RCTs including 281 adults (total 11 studies)
  • Tourette's syndrome: 2 RCTs including 36 adults (total 8 studies)
  • attention deficit hyperactivity disorder (ADHD): 1 RCT including 30 adults (total 3 studies)
  • post-traumatic stress disorder (PTSD): 1 RCT including 10 adults (total 12 studies)

Despite the large number of trails focused on depression and anxiety, medical cannabis was prescribed for this reason in only a couple of trials. In nearly all trials it had been prescribed for other reasons, mostly chronic pain. The studies just happened to look at the effect on mental health as well.

In nearly all trials, medical cannabis was given alongside other treatments for the condition, rather than being the sole treatment.

The trials used various types of medical cannabis, which was mostly taken by mouth, with only a couple of trials looking at vaporised or smoked cannabis.

Overall the quality of evidence was very low, with most trials carrying a risk of bias that could affect reliability of the results.

What were the basic results?

Nearly all evidence looked at medical-grade tetrahydrocannabinol and cannabidiol in combination (THC-CBD), with only a few looking at CBD alone or other types of medical cannabis.

The pooled results of 11 trials found that THC-CBD made no difference to depression symptoms. The pooled results of 7 trials suggested THC-CBD gave a small improvement in anxiety symptoms.

One psychosis trial suggested CBD may improve quality of life, while another suggested THC-CBD may worsen symptoms. Overall, none of the psychosis trials found that medical cannabis improved symptoms.

The 2 trials on Tourette's syndrome and the single trial on ADHD found no benefit from THC-CBD. The 1 trial in PTSD suggested THC-CBD made some difference to nightmare frequency and functioning, but it did not look at symptom scores or remission.

Overall, 10 trials (1,495 people) found that THC-CBD doubled the risk of side effects (odds-ratio 1.99, 95% confidence-interval (CI) 1.20 to 3.29), including the risk of worsening mental health symptoms. In 11 trials (1,621 adults), the chance of stopping treatment due to side effects was nearly tripled (2.78, 95% CI 1.59 to 4.86).

How did the researchers interpret the results?

The researchers conclude: "There is scarce evidence to suggest that cannabinoids improve depressive disorders and symptoms, anxiety disorders, ADHD, Tourette's syndrome, PTSD, or psychosis. There is very low-quality evidence that pharmaceutical THC (with or without CBD) leads to a small improvement in symptoms of anxiety among individuals with other medical conditions."

They suggest that there is insufficient evidence to provide guidance on the use of cannabinoids and that further high-quality studies are needed.


This is a valuable review that has gathered all the evidence to date on the use of medical cannabis in mental health conditions.

What it highlights most of all is the overwhelming poor quality of studies to date, which do not provide good evidence to support the use of medical cannabis for the treatment of mental health conditions. Studies were small and with variable methods, findings and risk of bias.

Even for anxiety – the only condition where they found a small improvement in symptoms – medical cannabis had been given to treat chronic pain or multiple sclerosis symptoms, not to treat anxiety. This makes the evidence on anxiety symptoms unreliable and of poor quality.

Other experts in the field have highlighted the need for quality evidence into medical cannabis, not only for mental health conditions, but for other medical conditions as well.

The National Institute for Health and Care Excellence (NICE) is in the process of producing guidance on the use of cannabis-based medical products.

It is important to see a doctor if you are suffering from symptoms of depression, anxiety or any other mental health condition. While there is currently no evidence to support the use of medical cannabis for these conditions, there are many safe and effective psychological therapies and medicines available that are backed by strong evidence.

NHS Attribution