More than one million Australians have used medicinal cannabis at some stage, mostly in the past five years, many for managing depression, anxiety and PTSD.
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 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.
Overprescription had the potential to do more harm than good, lead author Jack Wilson said.
"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," Dr 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 evidence in favour of cannabis use and its clinical administration, suggesting it could delay the implementation of more effective therapies for mental health patients.
"The evidence just falls short for using these medicines as treatment for those conditions," Dr Wilson said.
Australia legalised medicinal cannabis in 2016 and relaxed regulations governing its use 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 it, a 2024 study found
The Australian Medical Association and other medical peak bodies have raised concerns about the drug's largely unregulated growth, noting there is no limit to the concentration of tetrahydrocannabinol or THC, the psychoactive component in cannabis, in products on the market.
The vast majority of cannabis medications available to Australians have not been approved by the Therapeutic Goods Administration, the national drug regulator.
A review by the drug regulator received more than 750 submissions, many arguing for stronger product quality standards and more stringent labelling requirements.
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