Updated June 18.
Preliminary numbers show California's overdose deaths from the synthetic opioid fentanyl increased by 57% in 2017, according to the state's Opioid Overdose Surveillance Dashboard.
Fentanyl overdoses claimed 373 Californians, according to the Dashboard, an online tool that gathers data from the California Department of Public Health, the Office of Statewide Health Planning and Development, and the Controlled Substance Utilization Review and Evaluation System.
Fentanyl is up to 50 times more powerful than heroin and can be 100 times more potent than morphine. It's often added to other street drugs, and can overwhelm and kill someone who has no tolerance.
The number of overdose deaths last year could rise, however, according to Dr. Phillip Coffin of the San Francisco Department of Public Health. He said determinations on causes of death are still being closed in some 2017 cases.
Fentanyl has long been behind high rates of opioid deaths in the eastern U.S., but the potent drug has not brought the same crisis to the West, including California, Coffin said. But that's changing.
"We're definitely seeing a change in the market. We definitely are seeing fentanyl sold as fentanyl on the street. And that's new over the last couple of years," Coffin said.
The number of total opioid deaths -- including from heroin, prescription pain medication, morphine and fentanyl -- is down slightly from 2016.
Coffin attributes this to prevention efforts across the state, which include distribution of naloxone, a drug that reverses opioid overdoses, fentanyl test strips, and safer prescribing practices by doctors.
The number of overdose deaths from fentanyl has been changed to reflect the latest information released from the California Department of Public Health. Their original numbers erroneously stated that 746 people had died from fentanyl overdoses. The actual number is 373.