New novel synthetic opioids known as nitazenes may be more powerful than fentanyl and may require more doses of naloxone to reverse an overdose, according to a new study.

Researchers compared adults admitted to the emergency department who tested positive for nitazenes with adults who tested positive for fentanyl without nitazines. They found most of the patients who overdosed on nitazenes received two or more doses of naloxone, while most patients who overdosed on fentanyl received only a single dose of naloxone, CNN reports.

“Clinicians should be aware of these opioids in the drug supply so they are adequately prepared to care for these patients and anticipate needing to use multiple doses of naloxone,” the researchers wrote in the study. “In addition, to date there has been a lack of bystander education on repeat naloxone dosing.”

“This new study certainly adds to the body of concerning evidence that novel synthetic opioids, such as nitazenes, are really exacerbating the overdose crisis,” said Joe Friedman, a researcher at the University of California, Los Angeles David Geffen School of Medicine who studies the U.S. opioid crisis and was not involved in the new study. “It’s an understudied class of drugs, and one of the biggest problems we have is that we don’t actually systematically track drugs like nitazenes.”