Cannabis did not improve outcomes or reduce prescription opioid use among 1,514 Australians with noncancer pain, according to a recent report in Lancet Public Health.

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The subjects were referred from hundreds of pharmacies across the country. They had chronic pain for a median of 10 years – most commonly back pain and arthritis – and were on fentanyl, morphine, oxycodone, or other strong opioids. Subjects were interviewed and filled out questionnaires at baseline, then annually for 4 years.

Almost a quarter of patients reported using cannabis, mostly illicitly since most of the data were collected before Australia legalized medical marijuana in 2016. About 9% reported marijuana use in the previous month at baseline; 13% reported use in the past month at the final interview.

Overall, users rated the degree of relief they got from pain and pain-related distress as 7 out of 10, but the study findings did not support their impression.

At 4 years, cannabis users, compared with nonusers, reported greater pain severity (for daily or near-daily use: risk ratio, 1.17; 95% confidence interval, 1.03-1.32; for less frequent use: RR, 1.14; 95% CI, 1.01-1.29), more interference from pain in their daily lives (for daily or near-daily use: RR, 1.14; 95% CI, 1.03-1.26; for less frequent use: RR, 1.21; 95% CI, 1.09-1.35 ), less ability to cope with pain (for daily or near-daily use: RR, 0.98; 95% CI, 0.96-1.00; for less frequent use: RR, 0.97; 95% CI, 0.96-1.00), and greater generalized anxiety (for daily or near-daily use: RR, 1.10; 95% CI, 1.06-1.15; for less frequent use: RR, 1.07; 95% CI, 1.03-1.12). Results were adjusted for age, sex, pain duration, and other factors.

Pain severity scores on the 10-point Brief Pain Inventory, for instance, were 4.7 points at the end of the study among nonusers, compared with 5.3 among daily or near-daily users.

Few differences were reported in oral morphine equivalents between the groups. People who reported using marijuana 1-19 days a month were less likely to have discontinued opioids at 4 years (9%) than were those reporting no use (21%).

“Interest in the use of cannabis and cannabinoids to treat chronic noncancer pain is increasing because of their potential to reduce opioid dose requirements.” However, “we found no evidence that cannabis use improved patient outcomes” or that cannabis “exerted an opioid-sparing effect,” said the investigators, led by Gabrielle Campbell, PhD, of the National Drug and Alcohol Research Centre, Sydney.

The findings are not a slam dunk against cannabis for chronic pain. The investigators noted that people might have used cannabis because they had more pain to begin with and poorer coping mechanisms. Had they not been using marijuana, perhaps they would have been worse off.

However, “to date, evidence that cannabinoids are effective for chronic noncancer pain and aid in reducing opioid use is lacking. Large, well-designed clinical trials are required to evaluate in which patients cannabinoids might be effective in reducing pain severity, interference, and opioid doses,” they said.

Dr. Campbell and her associates cited several important limitations. Since cannabis use was primarily illicit, it’s unlikely that it was used under medical supervision. Also, the study only gauged frequency of use on a per-day basis. “We do not know if some people used once in a day or more than once. Likewise, we do not know what type of cannabis the participants used. ... This fact matters, as cannabis varies in strength and, as with any analgesia, the dose needs to be matched to the severity of pain experienced,” the team said.

The subjects were a median age of 58 years at baseline, and 56% were women. They had been prescribed a strong opioid for a median of 4 years at study entry and were on a median oral morphine equivalent dose of 75 mg/day, which fell to 57 mg/day at the study’s conclusion. About 62% of the subjects reported neuropathic pain.

The work was funded by the Australian government, and the National Health and Medical Research Council. Dr. Campbell reported grants from Reckitt Benckiser.

aotto@mdedge.com

SOURCE: Campbell G et al. Lancet Public Health. 2018 Jul. doi: 10.1016/S2468-2667(18)30110-5.

Cannabis falls short for chronic noncancer pain