Medical Cannabis Shows Limited Efficacy for Mental Health Disorders, Landmark Study Concludes
A major international review has determined that cannabis is not an effective treatment for common mental health conditions, challenging the growing global trend of patients using it for such purposes. Researchers from universities in Sydney, Brisbane, and Melbourne in Australia, along with Bath in England, conducted the largest and most comprehensive analysis to date on the use of cannabinoids—cannabis-based therapies—for treating substance misuse and mental health disorders.
Key Findings from the Review
The study, published in The Lancet Psychiatry, examined 54 randomised controlled trials involving 2,477 participants. It found "very little evidence for its efficacy" in addressing anxiety, anorexia nervosa, psychotic disorders, post-traumatic stress disorder (PTSD), or opioid use disorder. The researchers noted that there were no significant effects on outcomes associated with these conditions.
Additionally, the review highlighted insufficient evidence to assess whether cannabinoids help with attention deficit hyperactivity disorder (ADHD), bipolar disorder, obsessive-compulsive disorder, and tobacco use disorder. Notably, there was no evidence at all supporting its effectiveness for depression.
Context and Background
This review was prompted by the legalisation of cannabis as a treatment for mental health and substance misuse disorders in countries such as the US, the UK, Australia, and Canada. In the UK, medical cannabis was legalised in 2018, leading to a surge in clinics dispensing it. Evidence from these clinics indicates that mental illnesses like anxiety and depression are the primary reasons cited by patients, often followed by chronic pain, with some individuals suffering from both.
While the researchers acknowledged that medical cannabis products can reduce dependence on cannabis, alleviate symptoms of Tourette syndrome, and improve sleep for insomniacs, they emphasised that these benefits are based on limited or low-quality evidence. Similarly, findings suggesting cannabis might help reduce autistic traits in those with autism spectrum disorder were deemed to rely on "low" quality evidence.
Expert Opinions and Industry Response
Sir Robin Murray, a professor of psychiatric research at King's College London, criticised the cannabis industry, stating that many people are "misled into using cannabis to treat their problems." He described UK cannabis clinics as operating like "drug dealers for the middle class," arguing that therapeutic benefits are extremely limited and side effects are common.
In contrast, Mike Morgan-Giles, chief executive of the Cannabis Industry Council, defended the use of medical cannabis, asserting that real-world evidence, including findings from T21, shows it reduces symptoms for patients with anxiety and PTSD. He claimed the latest paper "adds more heat than light" and does not adequately reflect clinical prescribing practices in the UK.
Regulatory and Clinical Implications
The Advisory Council on the Misuse of Drugs, which advises UK ministers, is currently reviewing the implementation of the 2018 legalisation of cannabis-based products, including any "unintended consequences." Professor Owen Bowden-Jones, registrar of the Royal College of Psychiatrists, commented on the Lancet research, stating that it provides the clearest indication yet that the benefits of cannabis as a medicine may have been overstated for many conditions.
He emphasised that while these products show modest benefits for some addictions, they should not currently be offered for the many mental illnesses where no benefit was found. It is crucial, he added, for patients to receive accurate and transparent information to make informed decisions about their care and treatment.
Conclusion
Given the scarcity of robust evidence, the researchers concluded that the routine use of cannabinoids for treating mental disorders and substance use disorders is rarely justified. This review underscores the need for further high-quality research and careful consideration in clinical practice to ensure patient safety and effective treatment outcomes.
