A new report finds the latest wave of the opioid epidemic involves using multiple substances at the same time, combining fentanyl with either methamphetamine or cocaine, according to KFF Health News.

The report finds the United States is in the opioid epidemic’s “fourth wave,” which is making it more difficult to address the nation’s substance use problem. The findings come from Millennium Health, a specialty lab that provides testing services to monitor the use of prescription medications and illicit substances. The company analyzed 4.1 urine samples collected from January 2013 to December 2023 from people receiving some kind of treatment for substance use disorder.

Millennium found that almost 93% of fentanyl-positive urine samples contained additional substances. Nora Volkow, director of the National Institute on Drug Abuse, said the most concerning finding was the dramatic increase in the combination of meth and fentanyl use. “I never, ever would have thought this,” Volkow said.

The report says use of multiple substances complicates overdose treatments. For instance, while naloxone is widely available, there is no FDA-approved medication to treat stimulant overdoses.