In the last six days an abnormal number of people have overdosed on heroin, and three have died, officials say.
On average, four people overdose in the city per day, as compared to an estimated 78 overdoses over Tuesday and Wednesday.
"It's unlike anything we've seen before," said Hamilton County Commissioner Dennis Deters.
According to Cincinnati city manager Harry Black, the cause of the overdoses could be a powerful tranquilizer used on elephants mixed in with the drug. Carfentanil, a synthetic opioid, is about 100 times more potent than fentanyl, and is suspected in array of overdoses in several US states.
The crisis has left ill-equipped police and emergency responders drained.
Police in the Ohio city are asking for the public's help in identifying the source of the heroin batch that triggered the overdoses.