The results of one of the world's longest in-depth studies examining the relationship between cannabis use and chronic pain is raising doubts over whether cannabis reduces either pain severity or opioid use. The study, recently published in Lancet Public Health, is proving controversial as its findings contradict a growing body of research suggesting exactly the opposite.

The new Australian research, from the National Drug & Alcohol Research Center at UNSW Sydney, tracked over 1,500 subjects for four years. Participants suffered from chronic, non-cancer pain, and were all currently prescribed opioids as their primary pain management tool. Following a baseline interview and a three-month follow up, the subjects were reinterviewed every year for the following four years.

Using questionnaires and interviews all subjects self-reported their pain severity, mental well-being, and opioid and cannabis use. The results found no evidence that cannabis use reduces pain severity, in fact, the study reported that those using cannabis reported higher levels of both pain and anxiety compared to those not using the drug.

"At each assessment, participants who were using cannabis reported greater pain and anxiety, were coping less well with their pain, and reported that pain was interfering more in their life, compared to those not using cannabis," says Gabrielle Campbell, lead author on the study. "There was no clear evidence that cannabis led to reduced pain severity or pain interference or led participants to reduce their opioid use or dose."

The study is proving reasonably controversial in the field of medical marijuana research for its seemingly contradictory results when compared to other recent similar studies. While studies on the effects of cannabis for chronic pain have historically presented rather mixed results this seems to be due to the lack of specificity in many of these studies, as well as an inability to objectively determine a metric for pain severity.

Other research into more specific dosages and administrations of cannabis in relation to the drug's analgesic effects have found that low versus high dosing can make a key difference in the efficacy of its pain-modulating effects.

Another anachronistic element of this new study is the fact that while adjusted longitudinal analyses found no difference in pain severity between cannabis and non-cannabis users, the acute self-reporting of individual cannabis users suggested it was indeed effective for their pain. In fact, the study reports that the mean score for cannabis' efficacy on pain as self-reported by individual cannabis users was seven out of 10. The researchers hypothesize this odd inconsistency could possibly be due to cannabis' effects on other lifestyle aspects, such as improving sleep, which subsequently improves well being.

This data point importantly suggests that the effects of cannabis on the overall wellbeing of a patient suffering from chronic pain cannot be easily measured by simply trying to calculate acute pain severity.

A systematic review of all the current research by the National Academies of Sciences in 2017 concluded there is, "substantial evidence that cannabis is an effective treatment for chronic pain in adults." This review did note, however, that variations in different clinical study conclusions reflected the assortment of different doses and routes of administration, suggesting more specific work needed to be done to accurately find effective consumption methods.

One of the most significant limitations of this new study stems from the fact that for the majority of this Australian study, medicinal cannabis was illegal. This means the study collected no data on how subjects were consuming the cannabis or what form it took. David Caldicott, a researcher from Australian National University, even suggested in an interview with Buzzfeed, that all this study proves is that medical marijuana in an illicit market doesn't work.

"This paper shows that an unregulated market doesn't appear to work, but it certainly doesn't prove that medicinal cannabinoids don't work for pain,' says Caldicott.

It is also very significant to note the overall numbers of cannabis users versus non-cannabis users in the study seems to be markedly out of balance. At the final four-year follow up point there were a total of 1,217 subjects still in contact with the researchers. Of that number, only six percent (or 79 subjects) reported daily or near-daily cannabis use, and only 16 percent reported using cannabis at any point over the past 12 months.

In Australia, medical cannabis usage was only approved for a very limited number of conditions in 2016. While the United States and Canada are moving to more broadly allow recreational usages of the drug, Australia is moving much more slowly and conservatively. Despite the disappointing limitations of this study, and its conclusions being somewhat inconsistent with broader international research, it may slow down further medical cannabis legislation in the country over the coming years.

The study was published in the journal The Lancet Public Health.