Comprehensive Review Finds Limited Evidence for Cannabis in Treating Most Mental Health Conditions

Comprehensive Review Finds Minimal High-Quality Evidence for Cannabis in Treating Mental Health and Substance-Use Disorders

A recent comprehensive review of numerous studies indicates minimal high-quality evidence supporting the effectiveness of cannabis and cannabis-based medicines in treating most mental health and substance-use disorders. Published in The Lancet Psychiatry, the analysis synthesizes findings from 54 randomized controlled trials conducted over 45 years, involving nearly 2,500 participants. These findings emerge amid increasing global use of medical cannabis, often for mental health conditions.

Key Findings from the Review

The review, led by Jack Wilson of the University of Sydney, found no evidence to support cannabis as an effective treatment for several frequently cited reasons for medical marijuana use. Specifically, no significant benefit was identified for:

• Anxiety disorders
• Depression
• Post-traumatic stress disorder (PTSD)
• Anorexia nervosa
• Bipolar disorder
• Obsessive-compulsive disorder (OCD)
• Psychotic disorders, including schizophrenia
• Opioid-use disorder
• Attention deficit hyperactivity disorder (ADHD)
• Tobacco use disorder

The study authors also noted a significant gap in the evidence base, finding no randomized controlled trials that specifically evaluated cannabinoids for depression.

Conditions with Limited or Low-Quality Evidence

For a limited number of conditions, the review suggested potential benefits, though often with caveats regarding the quality of evidence:

• Cannabis-use disorder: A combination of cannabidiol (CBD) and delta-9-tetrahydrocannabinol (THC) was associated with reduced cannabis withdrawal symptoms and lower cannabis consumption.
• Tourette's syndrome: Cannabinoids were linked to reductions in tic severity.
• Autism spectrum disorder: Reductions in some autistic traits were observed, though this was based on "low-quality" evidence.
• Insomnia: Increased sleep time was noted in patients treated with cannabis medicines, but this data was also deemed "low quality."

Jack Wilson stated that, given the scarcity of evidence, the routine use of cannabinoids for the treatment of mental disorders and substance use disorders is currently rarely justified.

Potential Risks and Concerns

Experts highlight several potential risks associated with the increasing use of cannabis, particularly potent products, for mental health:

• Increased THC Potency: The psychoactive component, THC, has significantly increased in cannabis products over decades, from approximately 4% in the 1970s to an average of 18-20% currently, with some concentrates reaching 80%.
• Psychotic Disorders: Daily users of high-potency cannabis may face an increased risk, potentially six times higher, of developing psychotic disorders like schizophrenia or bipolar disorder compared to non-users.
• Vulnerable Populations: Use during pregnancy, adolescence, and young adulthood may interfere with brain development. Heavy use by teens and young adults with mood disorders has been linked to an increased risk of self-harm, suicide attempts, and death.
• Worsening Conditions: For individuals at high risk of bipolar or psychotic disorders, marijuana use may increase the risk of developing such conditions. Using cannabis after the onset of a mental condition may worsen cognition and increase the chance of relapse.
• Addiction: Approximately 3 in 10 marijuana users are reported to have cannabis use disorder, characterized by dependence and withdrawal symptoms such as irritability, restlessness, and sleep difficulties upon cessation.
• Other Side Effects: Participants in reviewed studies experienced significantly more side effects compared to placebo groups. One study also noted that medical marijuana intensified cocaine cravings in individuals with dependency.

Dr. Devan Kansagara of Oregon Health and Science University emphasized the importance of dose, suggesting that high THC-containing products carry more risks, especially for individuals with serious mental illness, and advised opting for lower-dose THC products to balance potential benefits and harms.

Research Challenges and Expert Perspectives

Researchers acknowledge difficulties in conducting and funding well-controlled trials for cannabis, partly attributed to its Schedule 1 drug classification by the DEA, which has historically limited large-scale investment.

Ryan Vandrey, a professor at Johns Hopkins University School of Medicine, noted that reviews of this nature have limitations, such as consolidating data from diverse products, doses, administration routes, and patient populations, and often excluding long-term observational studies. Vandrey mentioned that some of his own research, not included in The Lancet Psychiatry analysis, has indicated clinical benefits for certain patients with anxiety and depression, while acknowledging variable outcomes.

Ziva Cooper, director of the UCLA Center for Cannabis and Cannabinoids, pointed out that the review might overlook the distinct effects of specific cannabis compounds, such as cannabidiol (CBD), which has shown promise for anxiety. She advocated for integrating various types of data beyond placebo-controlled studies.

Regulatory and Clinical Context

Despite the limited evidence, the use of medical and recreational marijuana for mental health purposes is increasing in countries where it has been legalized, including the US, UK, Australia, and Canada. Mental illnesses like anxiety and depression are frequently cited by patients as primary reasons for seeking treatment.

Medical bodies, including the Australian Medical Association (AMA) and the Royal College of Psychiatrists, have raised concerns. Dr. McMullen, President of the AMA, stated that the study highlights the necessity for more robust regulatory oversight and stronger prescribing safeguards. Sir Robin Murray, a professor of psychiatric research at King’s College London, noted that many individuals are led to use cannabis for treatment partly by claims from the cannabis industry.

Mike Morgan-Giles, chief executive of the Cannabis Industry Council, asserted that "real-world evidence" demonstrates medical cannabis can reduce symptoms for patients with anxiety and PTSD. Cannabis Council Australia called for a comprehensive real-world evidence data registry to formally evaluate the safety, quality, and efficacy of medicinal cannabis.

The Therapeutic Goods Administration (TGA) in Australia has not approved over 99% of medicinal cannabis products prescribed there, and the Australian Health Practitioner Regulation Agency advises doctors to prescribe cannabis sparingly, as a last resort.

Proven Alternatives for Mental Health

Experts suggest that proven methods are available for treating mental health concerns. These include pharmaceutical approaches, such as selective serotonin reuptinake inhibitors (SSRIs) for depression and anxiety, and psychotherapy, such as cognitive behavioral therapy (CBT), which is often combined with SSRIs.