The Drug Enforcement Agency has urged Congress to classify illicit fentanyl analogues as Schedule I drugs permanently, claiming they’re a significant danger to public health.

The DEA’s temporary classification of fentanyl and its analogues expires in February 2020, acting chief of operations Greg Cherundolo told the Senate Judiciary Committee last week. Without a permanent classification, the DEA would enter “relatively unknown territory,” he said.

The temporary scheduling of all fentanyl analogues has been a notable success since its implementation 15 months ago, Cherundolo said, citing the “substantially slowed” rate of new fentanyl-related substances entering the illicit market. Prior to the emergency scheduling order, new fentanyl analogues rapidly emerged whenever the DEA scheduled a fentanyl-like substance, he said.