Four drug overdoses last year in San Francisco were linked to xylazine, a powerful animal sedative that is known on the street as “tranq” and which was found mixed with fentanyl in the bodies of the victims.

The San Francisco Chronicle reports:

At least four drug overdose victims who died late last year had traces of a horrific new street drug mixed with fentanyl in their systems, according to city officials, evidence that the animal sedative colloquially known as “tranq” has begun to infiltrate the city’s drug supply.

While San Francisco’s Office of the Chief Medical Examiner only found low levels of the drug, xylazine, in four out of the 145 victims who were tested, officials in the Department of Public Health said the discovery is “concerning” and that they also expect to see an increase in its prevalence on the city’s streets. The department issued a health alert Thursday warning the public of the potential danger and said that it is “working to understand the extent” to which xylazine is in the city’s street drugs and “respond to it accordingly.”

The U.S. Drug Enforcement Administration (DEA) warns that xylazine is “commonly encountered in combination with fentanyl but has also been detected in mixtures containing cocaine, heroin, and a variety of other drugs.”

Xylazine has already had an impact in several cities on the East Coast, the Chronicle adds.