Deputies found the men, all in their 50s, unresponsive and gave them Narcan, the office said.
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Naloxone, known by the brand name Narcan, is used in emergencies to reverse opioid overdoses.
Three men in Northern California died of a suspected fentanyl overdose. (Fox News)
Three of the men were pronounced dead at the scene. The fourth was hospitalized but there was no immediate word on his condition.
SAN FRANCISCO APPROVES PLAN TO ALLOW POLICE ROBOTS TO USE DEADLY FORCE IN EMERGENCY SITUATIONS
The names of the victims weren't immediately released.
The four are believed to have overdosed on fentanyl, authorities said, although the Medical Examiner-Coroner’s Office will determine the official cause of death.
Fentanyl is a synthetic opioid 80 to 100 times more powerful than morphine. It has been blamed for thousands of deaths in California alone.