There is no evidence that medicinal cannabis is an effective treatment for conditions like depression, anxiety or PTSD, according to a major new review by researchers at the universities of Sydney, Melbourne, Brisbane and Bath.
The study, which is published in Lancet Psychiatry, is the largest ever to look at the safety and effectiveness of medicinal cannabis for mental health and substance use disorders.
Anxiety, depression, PTSD, ADHD, insomnia and sleep disorders account for six of the top ten reasons cannabis is prescribed, the researchers state. However, they found ‘little evidence’ that medicinal cannabis is an effective treatment for these conditions, and while most side effects were mild to moderate, ‘some serious questions about safety remain’.
The researchers studied more than 50 randomised control trials involving almost 2,500 participants over a 35-year period, looking at whether medical cannabis reduced or effectively treated a range of mental health disorders, as well as substance use disorders including cocaine and opioids. The most common cannabinoid being evaluated was CBD, followed by THC or a combination of both.
‘We found cannabis medicines were no more effective than a placebo at treating symptoms of psychotic disorders such as schizophrenia, anxiety, PTSD, anorexia or opioid use disorder,’ said Jack Wilson and Emily Stockings of the University of Sydney. There were however ‘promising findings’ that medicinal cannabis could be effective in reducing cannabis use among people with a cannabis use disorder. ‘While this may sound strange, the medicines largely consisted of an oil-based combination of CBD and THC that was taken orally. As these medicines reduce craving, patients may use less of their usual cannabis. So for people who regularly smoke high-THC cannabis, using medicinal cannabis instead may reduce their risk of related health problems, such as lung conditions,’ they say.
They urged caution, however, when interpreting the ‘positive’ findings. There was a high risk of bias in the studies as the intoxicating effects of THC could mean participants being aware they hadn’t been given a placebo. As well as being psychoactive, THC is also linked to short- and long-term harms, so while the ‘data alone seems to suggest cannabis medicines are relatively safe’, this may not be reflected in ‘real-world use.’ Although the average length of treatment in the studies was just five weeks, one review from 2024 found that a quarter of people prescribed medicinal cannabis went on to develop a cannabis use disorder – a similar rate to that for non-medical use.
‘We need more research on cannabis medicines, particularly for conditions with limited alternative treatments, and monitoring over longer periods,’ the researchers say, adding that for people who believe their medicinal cannabis is beneficial ‘our review does not mean to contradict your experience. However, we encourage you to regularly discuss your circumstances with a doctor, and if possible, consider alternative evidence-based treatments.’ Doctors in the UK have been able to prescribe medicinal cannabis since 2018.
‘It is concerning that the use of these treatments could delay or replace the use of more effective therapies,’ the review states. ‘Overall, given the scarcity of evidence for efficacy and greater risk of all-cause adverse outcomes, the routine use of these medicines for mental disorders and [substance use disorders] is rarely justified.’
The efficacy and safety of cannabinoids for the treatment of mental disorders and substance use disorders: a systematic review and meta-analysis is published in Lancet Psychiatry, available here