Just 15% of patients being treated for opioid use disorder in residential treatment centers receive medication-assisted treatment, a new study finds.

Expert widely agree that anti-addiction medications such as naltrexone, buprenorphine or methadone are the most effective way to treat opioid use disorder, Reuters reports. “Patients entering these facilities are paying for a very high level of care, but might not be receiving the gold standard of treatment,” said study leader Andrew Huhn of the Johns Hopkins University School of Medicine in Baltimore.

The researchers found access to medication-assisted treatment was most limited in states that had less coverage for residential treatment through Medicaid, and in states with greater restrictions on prescribing those medications.

“Medications for opioid use disorder have repeatedly been shown to reduce the rates of opioid relapse and overdose death, and residential facilities should be working towards expanding all treatment options to address the opioid crisis that is devastating this country,” Huhn told Reuters.