Addiction specialists caution against reading too much into a new study released this week that compares two popular medications for opioid addiction. This much-anticipated research is the largest study so far to directly compare the widely used treatment Suboxone with relative newcomer Vivitrol.

Researchers who compared the two drugs found them equally effective once treatment started. But there are fundamental differences in the way treatment begins, which makes these findings difficult to interpret.

Vivitrol, an injection of naltrexone that lasts 28 days, has gained a foothold among treatment providers, especially those working with the criminal justice system.

Until recently, no major study had compared it to Suboxone, a combination of buprenorphine and naloxone that is taken by mouth daily.

Now researchers have found the two medications to be equally effective at preventing relapse once patients start treatment, according to a study published Tuesday in The Lancet. A smaller, shorter study out of Norway that was published in October came to a similar conclusion.

But the Lancet study highlights a limitation for patients starting on extended-release naltrexone: Patients have to detox before receiving their first dose of Vivitrol. That requirement creates a significant barrier to beginning treatment, says Dr. Joshua Lee, associate professor at the NYU School of Medicine and lead author of the report.

"It's going to take a few days or a week or more to get them on naltrexone in the first place," he says. "And that detox hurdle does not exist for buprenorphine."

Still, the research indicates that it would be advisable for treatment providers to offer both medications, he said. "Relapse rates are extremely high if you don't get onto and continue a medication," said Lee.

The two medications work in very different ways. Buprenorphine (like another addiction medication, methadone) is a long-acting opioid that's taken daily. There are decades of research showing that it helps reduce cravings and prevent withdrawal symptoms.

The Lancet study looked at a combination of buprenorphine and naloxone, which reverses the effects of opioids and is designed to prevent users from injecting or snorting the medication. Naltrexone is an antagonist — it blocks receptors in the brain and prevents opioids from having any effect. Vivitrol, which is delivered as a monthly injection, was approved to treat opioid use disorder in 2010 and until recently, no studies comparing buprenorphine and Vivitrol had been published.

"We've had trials of each one, but not together," said Lee. The latest study followed 570 patients from inpatient detoxification centers. They were randomly assigned to one of the drugs for six months. "Once people were on either one, they did reasonably well over time," said Lee.

But because extended-release naltrexone can throw people into withdrawal if administered too soon after opioid use, patients must first go through detox — abstaining from drug use — which often causes debilitating flu-like symptoms for several days. More than a quarter of patients assigned to naltrexone didn't complete detox, and most of them relapsed.

Buprenorphine treatment doesn't require patients to go through detox. "So up front there's a clear clinical advantage," said Lee. "Buprenorphine products are clearly easier to use."

Alkermes, the company that manufactures Vivitrol, has heavily marketed its flagship product to nonmedical professionals. As NPR and Side Effects reported earlier this year, the company has targeted lawmakers, judges and other criminal justice officials — people who may be ideologically opposed to using opioids to treat opioid addiction — in order to boost sales of its drug.

Given the tendency for criminal justice officials to favor non-opioid treatment options, Dr. Camila Arnaudo, an addiction psychiatrist who teaches at the Indiana University School of Medicine in Indianapolis, said she worries people will oversimplify the results of the study. "I'm a little bit concerned about headlines that I've already seen," she said, many of which have indicated that the products are equally effective, leaving out the caveat that many people initially failed out of treatment with extended-release naltrexone. "I'm concerned that it's going to lead to policies where patients are shunted into treatment with extended-release naltrexone, which is more acceptable to the criminal justice system."

She cautions people against reading too much into any one study, and pointed again to the detox hurdle. "You're weeding out the less committed people," she said, potentially skewing the results in favor of extended-release naltrexone. She said some patients will do better on one drug or the other depending on the case.

"I think we can say that both are viable options for patients and they prevent opioid use," she said. "I think what we cannot say, though, is that they're equivalent based on this study."

She added that some of the study authors reported receiving financial support and consulting fees from Alkermes.

Vivitrol treatment is more expensive. Each Vivitrol shot costs more than $1,200, according to Medicaid data, while a monthly supply of Suboxone can cost a few hundred dollars, depending on the dose.

Some inpatient treatment centers may also be ideologically opposed to starting someone on buprenorphine, particularly after detox, said Lee, but his study shows that it can be an effective option even starting in an inpatient setting. And in any event, relapse rates are higher among people who don't use medications for their opioid addiction.

"Detox episodes are brief," he said. "They don't generally last in terms of how you look a week later."

He added that if patients enter treatment with hopes of getting on Vivitrol but can't make it through detox, they should be offered buprenorphine. The bottom line, he said, is that both medications should be widely available and offered to patients suffering from opioid addiction.

"We're not doing a good enough job in this country of getting people into treatment and offering them these types of medications," said Lee. "So were just going around undertreating the opiate epidemic."

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