Cannabis is not an effective treatment for opioid addiction, a new study of thousands of people being treated for opioid use disorder suggests.

Experts, publishing their results today in The American Journal of Drug and Alcohol Abuse, have found that cannabis is having no significant effect on peoples' use of opioids, taken outside of medical guidance.

The findings have substantial implications for U.S treatment programs, some of which still require patients to abstain from cannabis before they qualify for potentially life-saving treatment. This is based on the belief they are more likely to use opioids non-medically if they are using cannabis.

The opposing, and increasingly popular, viewpoint, that cannabis can help wean people with opioid use disorder off opioids, is also called into question in this new study.

Opioids are effective painkillers, but they can also be addictive, and the U.S. remains in the grip of an opioid use disorder crisis.

Around 120 people die a day from drug overdoses involving opioids (prescription, such as oxycodone, and non-prescription, such as heroin) and opioid use disorder and related deaths cost the US economy more than $1 trillion a year.

As cannabis gains popularity among individuals with opioid use disorder in the U.S., its medicinal use is now legally recognized in thirty-seven states and Washington D.C. While pain remains the most common reason for medical cannabis authorization (i.e., "medical cannabis registration card"), an increasing number of states are adding "alternatives to opioids" or "opioid-treatable disorders" to their lists of approved conditions. In certain states, this includes treatment for opioid use disorder.

The study's authors say this partly because the legalization of the recreational use of cannabis in many states means the drug is being perceived as being less harmful than in the past. Some cannabis dispensaries have promoted medicinal cannabis as a treatment for opioid use disorder.

It isn't clear, however, whether cannabis helps or hinders the treatment of opioid use disorder. Some studies have found it helps alleviate pain and opioid withdrawal, but others suggest it makes a return to opioids more likely.

"Clarifying how cannabis and opioids interact is crucial if we are to equip health care professionals to provide evidence-based addiction treatment, prevent overdose deaths and save lives," says researcher Gabriel Costa, of University of Ribeirão Preto in Brazil.

Costa, under the mentorship of Dr. Joao P. De Aquino, of Yale University, and colleagues, carried out a systematic review and meta-analysis of existing research on the influence of cannabis on non-medical opioid use.

The meta-analysis combined the results of ten longitudinal studies involving 8,367 individuals who were receiving medication (buprenorphine, methadone or naltrexone) to treat their opioid use disorder.

As part of this, over the course of an average of 10 months, individuals were monitored for their non-medical opioid use—including the use of opioids not prescribed to them, taking more opioids than prescribed, or using opioids without a prescription.

The study compared the frequency of this use between individuals who used cannabis, typically obtained from non-regulated sources, and those who did not use cannabis.

Results showed there to be no link between cannabis use and rates of non-medical opioid use.

"Overall, we found no significant association between cannabis and non-medical opioid use among patients receiving pharmacotherapies for opioid use disorder," states Costa.

"These findings neither confirm concerns about cannabis increasing non-medical opioid use in individuals being treated for opioid use disorder, nor do they endorse its efficacy in reducing non-medical opioid use."

The implications for opioid use disorder treatment programs are significant, adds Dr. De Aquino, who is a specialist in the treatment of persons with substance use disorders and co-occurring medical and psychiatric disorders.

He explains, "Our finding questions the ineffective practice of enforcing cannabis abstinence as a condition to offer life-saving medications for opioid use disorder.

"Our data suggests health care systems should instead adopt individualized treatment approaches which take into account each patient's circumstances.

"This would include assessing cannabis use disorder, a problematic pattern cannabis use that affects a person's well-being and ability to function, addressing pain management needs and treating co-occurring psychiatric conditions, such as depression and anxiety."

Dr. De Aquino adds that there have been very few experimental studies into cannabis and its constituent cannabinoids' ability to alleviate symptoms of opioid use disorder, and randomized placebo-controlled trials are needed to thoroughly assess its safety and effectiveness.

He says, "As high-potency synthetic opioids such fentanyl become increasingly available, it is of utmost importance that individuals with opioid use disorder have access to FDA-approved treatments.

"Methadone, buprenorphine, and extended-release intramuscular naltrexone—are known to be life-saving and are the cornerstone of opioid use disorder management."

Limitations include a lack of consistency in how the studies in systematic review and meta-analysis were conducted. This includes differences in how cannabis and opioid use were measured and variations in baseline opioid use status.

In addition, although the results are applicable to general cannabis use, they may not apply to individuals with cannabis use disorder.