MIAMI — A hazardous new synthetic drug originating in China is being blamed for 18 recent deaths in a single South Florida county, as police grapple with an inexpensive narcotic that causes exaggerated strength and dangerous paranoid hallucinations.

On Thursday, the Fort Lauderdale police killed a man, reportedly high on the man-made street drug, alpha-PVP, known more commonly as flakka, who had held a woman hostage with a knife to her throat.

The shooting of Javoris Washington, 29, was the latest in a series of volatile episodes that the police in South Florida have faced with highly aggressive drug users. Law enforcement agencies have had difficulty tamping down a surge in synthetic drugs, which were banned after becoming popular in clubs five years ago only to re-emerge deadlier than ever under new formulations. As soon as legislation catches up with the latest craze, manufacturers design a new drug to take its place, federal and local law enforcement agencies say.

In Broward County, which includes Fort Lauderdale and is considered ground zero for the new drug, there have been 18 flakka-related fatalities since September, the chief medical examiner there said.