ISO hits San Francisco streets: synthetic opioid 20x stronger than fentanyl kills

A new synthetic opioid called ISO has claimed its first life in San Francisco and is flooding the Tenderloin district. Health officials warn it is undetectable by fentanyl test kits.

San Francisco's Tenderloin overrun by ISO, a synthetic opioid deadlier than fentanyl

San Francisco's drug crisis has a new front. NY Post Metro reports that a counterfeit opioid called isotonitazene — known on the streets as ISO — has begun circulating widely through the city's Tenderloin district, and health officials are warning it is roughly twenty times more potent than fentanyl.

The San Francisco Health Department confirmed in an April alert that ISO claimed its first known fatality in the city that month. The overdose death, announced April 23, involved ISO blended with a second synthetic opioid, cychlorphine. Neither substance registers on standard fentanyl test kits, officials noted — making the combination especially difficult to detect before use.

"Even a single pill can contain a lethal dose," city health officials stated in the alert.

Visible toll in the Tenderloin

Video footage filmed by Manhattan Institute writer Christopher Rufo and circulated online shows clusters of users hunched over or lying motionless on Tenderloin sidewalks. Rufo said a street dealer told him ISO is gaining ground fast.

"For the last ten years, it's been an arms race for increasing the strength of opioids, from heroin to fentanyl, and now, to ISO," Rufo said. "It's devastating California cities."

The Center for Forensic Science Research and Education identifies isotonitazene — also called Nitazine — as approximately 20 times more potent than fentanyl. It can be smoked, injected, or taken as a pill.

Federal agencies issue warnings

The federal Drug Enforcement Administration issued a fresh warning last month about ISO and related synthetic drugs that may be cut with fentanyl. The agency cautioned that users who overdose on these substances may not respond to naloxone, the standard overdose-reversal medication.

"These emerging synthetic drugs can be significantly more powerful than fentanyl and greatly increase the risk of suffering a fatal overdose," the DEA advisory stated.

The DEA first identified ISO in U.S. illicit drug seizures in 2019. Since then, the agency has logged more than 900 reports of the substance — 42 in 2019, a peak of 358 in 2021, and 43 in 2024, the most recent year with available data. A separate DEA bulletin issued in December 2025 described ISO as a "serious concern to the public safety."

ISO has no approved medical use and no industrial application.

Spread to Los Angeles remains limited — for now

Law enforcement officials in Los Angeles told NY Post Metro that ISO has not yet emerged as a significant threat in that city. One police source said, however, that illicit chemists there are increasingly mixing fentanyl with a range of other substances, including animal tranquilizers. Los Angeles health officials attribute just over half of all drug and alcohol deaths in the city to fentanyl. Police have recently intensified efforts to clear drug use from MacArthur Park in downtown LA.

Activists push for stronger police response

Jacqui Berlinn, co-founder of the San Francisco-based advocacy group Mothers Against Drug Addiction and Deaths, said the city's police force should be doing more to address conditions on the streets. She argued that ISO's combination of extreme potency and undetectability makes it uniquely dangerous.

"It's incredibly scary because it's taking the addicts unaware," Berlinn said.

San Francisco health officials have not yet detailed additional enforcement or harm-reduction measures specific to ISO beyond the April alert.

Source: NY Post Metro