Almost all counterfeit oxycodone pills tested by police labs in Rhode Island in 2022 contained fentanyl, HealthDay reports.

Most of the pills also contained xylazine, another potentially lethal substance, Brown University researchers report in JAMA. Both fentanyl and xylazine increase the chance of overdose and the risk the overdose will be fatal, experts say.

Pat Aussem of Partnership to End Addiction told HealthDay that xylazine is a depressant that slows brain activity and causes extreme drowsiness. “It can result in slowed breathing and heart rate in people who use it, as well as dangerously low blood pressure,” she said. “The risk of overdose is amplified when combined with other substances like fentanyl.”

The researchers obtained state forensic drug chemistry lab test results for 1,176 counterfeit pills seized by law enforcement in Rhode Island between early 2017 and the end of 2022. Most of the counterfeit pills were sold as oxycodone, alprazolam (Xanax) or amphetamines. Almost all of the counterfeit oxycodone pills contained fentanyl, compared with about 10% between 2017 and 2020.

In contrast, fentanyl was found in 2.6% of the fake Xanax pills. None of the counterfeit amphetamine pills contained fentanyl.