A street drug that combines fentanyl and a new synthetic opioid is being sold illegally as the prescription painkiller Norco, according to a new report. Researchers caution that the street version is much stronger and more hazardous than the real medication.

The illegal version of Norco looks very similar to brand-name Norco, according to Dr. Patil Armenian of the University of California, San Francisco. She reported the case of a woman who took the illegal version of Norco in the Annals of Emergency Medicine.

Legal Norco contains acetaminophen and hydrocodone, HealthDay reports. The illegal version has led to an unexpected cluster of fentanyl deaths in California this spring, Armenian said.