The opioid use disorder (OUD) epidemic in the United States is a top concern among healthcare professionals, but there’s little research investigating what works to treat it.

The first study assessing the most effective treatments for OUD, conducted through Massachusetts General Hospital and Harvard Medical School, finds that 1.8 million adults in the US have recovered from it. (For context, a 2016 study found that 2.1 million people that year struggled with a substance abuse disorder related to prescription opioids, according to the National Institute on Drug Abuse.

To come to this conclusion, researchers analyzed data from the 2017 National Recovery Survey based on people aged 18 and up who answered “yes” to the question “Did you used to have a problem with drugs or alcohol but no longer do?” Looking at the prevalence of both alcohol and drug abuse recovery, they focused on the different rates with which each group used treatment and recovery services and their reported state of psychological well-being both within early recovery (the first year) and mid-recovery (between one and five years).

The most significant finding: Individuals suffering from OUD were more likely than those with alcoholism to have used formal addiction treatment, drugs to prevent cravings or relapse (such as methadone, buprenorphine, or naltrexone), recovery support services, and 12-step programs, such as Narcotics Anonymous.

“Essentially, those who resolved an opioid problem in mid-recovery were four times as likely to have ever used pharmacotherapies, two-and-a-half times more likely to have used formal treatment, and about two times more likely to use recovery support services and mutual help organizations compared with individuals who reported resolving an alcohol problem and were in mid-recovery,” said study co-author Lauren A. Hoffman, Ph.D., in a statement.

“We didn’t find those differences in the first year, and this is important because taken together, it suggests that individuals with an opioid problem might require additional treatment or additional resources to achieve longer and more stable recovery duration,” she added.

When looking at measures of psychological well-being, researchers found levels of self-esteem were higher in the opioid group than in the alcohol group in early recovery. But self-esteem levels among the opioid group were lower than the alcohol group in mid-recovery.

Hoffman speculates that this switch may be because lower self-esteem in mid-recovery is related to the “extra challenges of opioid recovery, including longer duration or greater use of treatment and recovery services relative to alcohol recovery, more frequent relapses and societal attitudes about opioids.”

That’s why healthcare providers need to remember that people suffering from OUD likely feel the social stigma and are less likely to talk about their recovery, which can make them feel isolated, researchers say.