Researchers looked at the impact of medical cannabis on six different conditions: depression, anxiety, Tourette’s syndrome, post-traumatic stress disorder (PTSD), attention-deficit hyperactivity disorder (ADHD) and psychosis.
The large-scale study, published in the Lancet Psychiatry journal, analysed data from 83 previous studies conducted over the past 40 years involving more than 3,000 people.
The researchers said there is “inadequate evidence” that cannabinoids (the compounds in cannabis) relieve symptoms associated with the six conditions, adding that prescriptions of medical cannabis for these conditions “cannot be justified based on the current evidence”.
They also found evidence that some medicinal cannabinoids worsened negative symptoms for people with psychosis.
Cannabinoids, the active chemicals in cannabis, cause drug-like effects throughout the body, including the central nervous system and the immune system. The researchers conducted the review because medicinal cannabinoids – plus the cannabis-derived ingredients tetrahydrocannabinol (THC) and cannabidiol (CBD) – have been “suggested to have a therapeutic role in certain mental disorders”.
Medical cannabis is currently only prescribed on the NHS for the treatment of severe forms of epilepsy or to adults with vomiting or nausea caused by chemotherapy. However, CBD wellness products, such as CBD oil, are available on the high street without prescription and are becoming increasingly popular. Some advocates claim these products can ease anxiety.
Experts have previously told HuffPost UK any benefit experienced via non-prescription CBD wellness products is likely due to “a very substantial placebo response”, due to the fact non-prescription CBD typically contains very low levels of the active ingredients.
The latest study goes further than this, highlighting that even in higher concentrations, there is “little evidence” that medicinal cannabinoids, THC and CBD, can ease mental health issues.
The study does suggest there is “very low quality evidence” that pharmaceutical-grade THC may lead to a “small improvement” in symptoms of anxiety in individuals with multiple sclerosis or chronic pain. However, this may have been due to improvements in the primary medical condition, the researchers said.
Cannabinoids are increasingly being made available for medicinal purposes in countries like the United States, Australia and Canada, including for the treatment of mental health issues. However, the researchers have raised concerns around the adverse effects of this availability. As well as little evidence that medical cannabis improves mental health conditions, there is a large body of evidence that indicates smoking non-medical cannabis recreationally can increase the occurrence of depression, anxiety, and psychotic symptoms, they said.
Lead author Professor Louisa Degenhardt, of the National Drug and Alcohol Research Centre (NDARC) at UNSW Sydney, Australia, said: “Our 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.”