Tests have confirmed carfentanil in the bodies of eight recent overdose victims in the Cincinnati area of Ohio, and more cases are suspected.
It is a drug used to sedate elephants and can be thousands of times stronger than morphine.
Experts said an amount smaller than a snowflake could kill a person.
Hamilton county coroner Dr Lakshmi Sammarco said the sudden surge in overdoses raised concerns that dealers wanted to see what would happen in a community when they mixed carfentanil with heroin, or sold it outright.
She said Cincinnati was being used as a “test tube” by dealers.
Tom Synan, the chief of Hamilton County Regional Narcotics Unit, said local authorities should declare a public health emergency.
Nearly 300 overdoses have been reported in the Cincinnati area since 19 August, with 174 reported in one six-day period.
There have also been increases in overdoses recorded in West Virginia, Kentucky and Indiana.
US Drug Enforcement Administration officials have said they believe much of the carfentanil is being shipped from China to Mexico, where traffickers are mixing it with heroin and other drugs such as the painkiller fentanyl.
Fentanyl can be 50 times stronger than heroin, and carfentanil is as much as 100 times more potent than fentanyl.
News Source SkyNews