Cannabis should not be considered an effective long-term strategy for reducing opioid use, according to a new study released amid the global shift towards cannabis legalisation.
A study led by the University of Sydney, published on Wednesday in the American Journal of Psychiatry, followed 615 people with heroin dependence, many of whom also used cannabis, over 20 years.
The lead author of the study, Dr Jack Wilson, from the Matilda Centre for Research in Mental Health and Substance Use, said the study was the longest to have ever looked at the relationship between cannabis and illicit opioid use.
“There are claims that cannabis may help people with opioid use disorders because cannabis may be used as a method of reducing pain or helping people manage their opioid withdrawal symptoms.”
However, the study found there is no evidence cannabis reduces long-term illicit opioids use.
In 2001, the researchers recruited and interviewed a group of people with heroin dependence from Australia and then followed them up on six occasions over the next 20 years: at three months, 12 months, 24 months, 36 months, 10-11 years, and 18-20 years later.
About two-thirds were using cannabis when they were first recruited in 2001, which is representative of use among people with opioid-use dependence, Wilson said. Of those who were using cannabis, “throughout 20 years, we did not find there was a clear relationship between the use of these substances. Those using cannabis, it didn’t seem to help them with their opioid use.”
Wilson said the results showed that clinicians and policymakers should not rely on cannabis to reduce problematic opioid use in the context of growing access to cannabis products, with the global shift towards legalisation and recognition as a therapeutic product.
The non-medical use of opioids can lead to opioid dependence and other health problems, including difficulties breathing and death in the case of overdose. The burden of opioids in Australia and North America is significant, with opioid use responsible for more death and disability than any other illicit drug, Wilson said.
The cohort of people with heroin dependence in the study were typical of people with opioid use disorders, with poor physical and mental health, unstable housing, more likely to be men and went in and out of different treatment settings, Wilson said.
“Opioid use disorders are complex and unlikely to be resolved by a single treatment,” Wilson said. “The best way to support them is evidence-based, holistic approaches that look at the bigger picture and include physical, psychological and pharmacotherapy therapies.”
“Even though we found no evidence to suggest a long-term relationship between cannabis and opioid use, I wouldn’t make any conclusions around the efficacy of cannabis as a treatment for pain,” Wilson cautioned.
Previous studies have not extended beyond several years, with most restricted to a single follow-up point.
Mark Connor, a professor of Pharmacology at Macquarie Medical School, who wasn’t involved in the study, lauded the successful follow-up rates. “It’s a pretty amazing achievement to be able to speak to two-thirds of people you spoke to 20 years ago who were entering a heroin treatment program.”
Connor said while “people would like to think that a potentially less harmful drug like cannabis might be able to substitute for a potentially more harmful opioid, there is no evidence of that from this study and that doesn’t surprise me at all”.
“It’s like a naturalistic study; there’s no experiment here. They’re not asking people to do anything. They’re not trying to change their behaviour.”
Prof Suzanne Nielsen, the deputy director of the Monash Addiction Research Centre in Melbourne, said the findings were consistent with other new research showing cannabis use does not have a role in reducing opioid-use disorder.
She said the idea that it ever did came from low-quality research studies when cannabis was first legalised in the US. These suggested there was a correlation in reduction in opioid overdose, but when the studies were repeated they found a reversal of the association and that the laws actually created greater opioid-related harm, Nielsen addded.
She said because the initial study was taken as evidence cannabis would reduce opioid use, at least one state added opioid use disorder as a reason that people could prescribe medical cannabis.
