More than one million Australians have used medicinal cannabis at some stage, mostly in the past five years, for managing conditions including depression, anxiety, PTSD and substance use disorders.
But a meta-analysis of peer-reviewed research, including 54 randomised controlled trials, since 1980 suggests there is little evidence the drug is effective for treating the conditions.
While evidence showed that medicinal cannabis was helpful for treating autism, Tourette's syndrome and insomnia, the Australian researchers concluded routine cannabis use for mental health patients was rarely justified.
The study was published in The Lancet Psychiatry on Tuesday.
"We now know that the risk of developing a cannabis-use disorder among those who are using cannabis for medical use, compared to those using cannabis non-medically, is just as high," lead author Jack Wilson told AAP.
"It can also make your mental health symptoms worse."
The study, led by researchers at the University of Sydney, the University of Queensland and Monash, found medical marijuana intensified cocaine cravings among those with a dependency.
People who took cannabis products in the reviewed studies experienced significantly more side effects compared with people in the trials who had a placebo, but serious adverse events and study withdrawals did not differ between groups.
The research pointed to a gap between the scarcity of evidence in favour of cannabis use and its clinical administration, suggesting the drug could delay more effective therapies for mental health patients.
"The evidence just falls short for using these medicines as treatment for those conditions," Dr Wilson said.
But the peak body for medicinal cannabis prescribers says the review's findings do not indicate a death knell for the industry.
"A comprehensive real-world evidence data registry is needed to enable ongoing, formal evaluation of safety, quality and efficacy of medicinal cannabis," a Cannabis Council Australia spokeswoman said.
While many patients report significant benefits from cannabis treatment, there is still not enough data to know for sure, drug safety group Penington said.
"Medicinal cannabis regulations can and should be improved to support better patient health and safety outcomes, but reforms must be pursued cautiously," chief executive John Ryan said.
Australia legalised medicinal cannabis in 2016 and relaxed regulations governing its prescription in 2021 to allow its expansion to treat more than 250 health conditions.
That is consistent with worldwide trends, with recreational and medical use rising in recent decades as more nations legalise cannabis, a 2024 study found.
More than 99 per cent of all medicinal cannabis products prescribed in Australia have not been approved by the Therapeutic Goods Administration, the national drug regulator.
The Australian Medical Association and other medical peak bodies have sounded the alarm on the drug's largely unregulated growth, noting there is no limit to the concentration of tetrahydrocannabinol or THC, the psychoactive component in cannabis, in unregistered products.
The Australian Health Practitioner Regulation Agency, which oversees medical professionals, advises doctors to prescribe cannabis sparingly and out of necessity, as a last resort after other treatments have failed.
"Medicinal cannabis should only be prescribed when a practitioner has thoroughly assessed that it is the best treatment option for their patient – the same as with any other medicine," a spokeswoman for the agency said.
The Lancet Psychiatry study emphasised the high-THC medicines Australians were accessing could exacerbate cannabis dependence and other long-term harms beyond the scope of their research.
The chemical has been linked to anxiety, depression, psychosis and suicidal thoughts.
A consultation by the drug regulator last year received more than 750 submissions, many arguing for stronger product quality standards and more stringent labelling requirements.
A spokesman for the Therapeutic Goods Administration said investigations into how current regulations worked in practice were ongoing.
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