The study, one of the largest and most in depth ever done on the drug's medical use, found cannabis does not cut pain, nor does it help sufferers replace opioids. And users seem to suffer higher levels of anxiety overall.
Cannabis has proven medical benefits in treating drug-resistant epilepsy in children, chemotherapy-induced nausea and multiple sclerosis.
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The participants all started with similar levels of pain but as the study went, on those who turned to cannabis reported higher levels of pain and anxiety.
"We found no evidence that cannabis use improved patient outcomes," the researchers wrote in Lancet.
The study raises questions for several stakeholders including political leaders such as premier of the Australian state of Victoria, Daniel Andrews, who have thrown their weight behind legalising the drug, and Australia's medical cannabis entrepreneurs, who have already made sharemarket fortunes before getting a drug to market.
"We try to moderate expectations people have around managing pain," says Professor Michael Farrell, director of the National Drug and Alcohol Research Centre and a senior author on the Lancet paper.
"There has been a certain amount of media coverage suggesting cannabis is the answer, and people who otherwise would not have contemplated going near illicit cannabis have been exploring it.
"I think the sell on the impact of cannabinoids has been a pretty hard sell and we do need to approach it with caution."
For the study, researchers randomly selected a group of 1514 people who were already participating in another long-term study on using prescription opioids to treat chronic pain.
Over four years, researchers repeatedly surveyed the participants about their pain and how they were treating it.
The survey starkly illustrates the community's increasing interest in medical cannabis. By the end of the study, most members of the group were either using cannabis or would if they could get it.
But it did not seem to work.
The participants all started with similar levels of pain but as the study went, on those who turned to cannabis reported higher levels of pain and anxiety.
It did not help them cut their opiod use at all.
However, the results are not straightforward.
Most of the cannabis users reported they personally felt it worked well to treat their pain - despite also reporting higher levels of pain.
It's possible, the researchers say, these people may have sought cannabis because their pain worsened over the course of the study, or they felt conventional drugs weren't working.
Cannabis is also a powerful sedative; a good night's sleep can be confused with effective pain treatment.
Or, it could simply be that cannabis does not work for chronic pain.
"It's really difficult to disentangle the reasons," says lead author Dr Gabrielle Campbell, a fellow at NDARC.
There are also issues of quality. The study asked people if they had used cannabis to control their pain, but not the form, strain or source.
Medically prescribed cannabis will be much more pure and will use specific strains.
"For people who aren't getting relief, my first question would be how: many strains have you tried?"
says Deb Lynch, president of the Medical Cannabis Users Association. "There is a lot of trial and error in the beginning to find one that works for you."
Lynch uses cannabis - sourced illegally - to treat a range of conditions including chronic pain.
"It has given me back my life. Prior to starting on the oil treatments, I was on high doses of multiple opiates. I was in bed wanting to kill myself and my pain wasn't being addressed. Now I'm running all over the country."
The study won't be good news to Australia's burgeoning field of medicinal cannabis producers, such as AusCann, a sharemarket darling with plans to launch its first product targeting chronic pain in early 2019.
"There is a mixed bag of trials on medical cannabis, using differing formulations and testing on various conditions" said Dr Stewart Washer, the company's co-founder. "But well-designed trials get a 50 per cent reduction in pain. It's huge.
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