The decline in opioid-related overdose deaths in 2018 was likely due to a decrease in the availability of the drug carfentanil after China changed its drug manufacturing regulations, according to a new study.

Researchers cross-referenced drug overdose data from the Centers for Disease Control and Prevention with information from the U.S. Drug Enforcement Administration about the types of drugs seized and submitted to state crime labs.

They found a drop in overdose deaths in Florida, Kentucky, Michigan, Ohio and Pennsylvania from 2017 to 2018 coincided with a drop in seizures of the opioid carfentanil by law enforcement in the same period, UPI reports. The decline in overdose deaths in these states accounted for almost all of the total national decline in 2018, the researchers wrote in Addiction.

China added carfentanil to its list of controlled substances in 2017, and the U.S. supply eventually dried up at the same time the U.S. overdose deaths declined in 2018.

“The U.S. has not bent the curve on the drug overdose epidemic,” lead author Hawre Jalal, M.D., Ph.D., of the University of Pittsburgh said in a news release. “We are concerned that policymakers may have interpreted the one-year downturn in 2018 as evidence for an especially effective national response or the start of a long-term trend. Unfortunately, that isn’t supported by the data.”