When San Francisco police seized seven kilos of powder-filled baggies containing the deadly opioid fentanyl last week, the city’s police chief warned the bust contained “enough lethal overdoses to wipe out San Francisco’s population four times over.”

But, according to a report published in The Guardian, drug addiction experts say the haul may represent just a tiny fraction of the massive volume of the powerful synthetic drug that is flooding California, after being mostly an east coast phenomenon for years.

The evidence is in the rapidly surging death rates. The number of deaths from fentanyl overdoses jumped by more than 2100% in California in five years, state figures show. Overdoses of synthetic opioids (mostly fentanyl) killed nearly 4,000 residents in the state last year, according to the most recent estimate from the US Centers for Disease Control and Prevention.

In San Francisco, drug users are dying at a rate of nearly two a day, many on the streets of the city’s Tenderloin District.

In San Diego, fentanyl is coursing through the homeless population, according to experts and recent media reports. Santa Clara county saw the number of fentanyl deaths more than double last year, KQED reported, with victims on average younger than in previous years.

“Fentanyl has moved west,” said Dr Daniel Ciccarone, a professor specializing in addiction medicine at the University of California, San Francisco. Ciccarone said the lab-made drug was barely seen in western states before 2017. Instead, he said, it used to be distributed by drug trafficking networks supplying the east coast, who often slipped it into heroin supplies without telling users.

In a paper released this month, Ciccarone describes the explosion of accidental overdose deaths occurring west of the Mississippi as part of a “fourth wave” of the opioid crisis.

In California, the drug is being sold under its own name, as powders or tablets. It’s also being mixed with stimulants, like methamphetamines.

Fentanyl is so powerful that a quantity small enough to fit under a fingernail can be deadly within minutes. Dr Aimee Moulin, a professor of emergency medicine at the University of California, Davis medical center in Sacramento, said she was seeing adolescents as young as 13 overdose on counterfeit opioid pills available for home delivery over the internet.

“The potency is so high that a decimal point difference in the concentration can be lethal,” she said.

Fentanyl is an attractive product for drug cartels because it can be cheaply manufactured in foreign clandestine laboratories and substituted for more expensive drugs like the white-powdered heroin commonly sold on the east coast or pressed into counterfeit pills sold as OxyContin or Percocet, according to the 2020 National Drug Threat Assessment from the US Drug Enforcement Administration.

The Sinaloa cartel and Jalisco New Generation cartel from Mexico have been taking over production and distribution from prior sources, which included China, the report said.