Deaths from a new synthetic opioid known as “iso” are increasing, HealthDay reports.

Iso, short for isotonitazene, is the cause of 40 to 50 overdose deaths a month in the United States, up from about six per month last summer, according to Dr. Antonio De Filippo, an addiction specialist and Medical Director at Delphi Behavioral Health Group in Fort Lauderdale, Florida.

The drug is a synthetic form of a highly addictive opioid called etonitazene, which is a controlled substance in the United States. Iso is legal at the national level because it has a slightly different chemical structure.

“From a purchasing standpoint, a lot of it is being sold online legally by Chinese suppliers offering bulk deals,” said Pat Aussem, Director of Clinical Content and Development at Center on Addiction in New York City. “The U.S. challenged China to crack down on fentanyl. Chemists there are looking for other ways to fill the demand.”