San Francisco’s Drug Supply Just Got 20 Times Deadlier Than Fentanyl.

A decade into the fentanyl catastrophe that has killed more than 3,000 people in San Francisco, the city’s drug supply has mutated again — and the new ingredient is worse. Forensic chemists have confirmed isotonitazene, a synthetic “nitazene” opioid roughly 20 times more potent than fentanyl, circulating on Tenderloin streets, often blended with a second novel opioid called cychlorphine.

The drugs do not show up on the fentanyl test strips the city has handed out by the hundreds of thousands. They can shrug off a standard dose of Narcan. And they arrived in a city whose decade-long answer to addiction was built almost entirely around the assumption that the poison on the street was fentanyl — not something an order of magnitude stronger.

San Francisco recorded the first confirmed overdose death tied to the ISO-cychlorphine blend in April 2026. Public-health officials warn that a single counterfeit pill can carry a lethal dose, and that users have no way of knowing it is in their supply.

• ~20× — more potent than fentanyl — isotonitazene (ISO), the nitazene now found in San Francisco's drug supply · Source: Center for Forensic Science Research & Education
• 621 — drug-overdose deaths in San Francisco in 2025 — second-highest of any U.S. county, behind only Baltimore · Source: SF OCME / KRON4 / CDC
• Apr 23, 2026 — date SF announced its first fatal overdose linked to the ISO + cychlorphine blend; Narcan may not reverse it · Source: KRON4 / SF DPH

Isotonitazene belongs to the nitazene family — synthetic opioids first cooked up in 1950s pharmaceutical research and never approved for medical use. The Center for Forensic Science Research & Education puts ISO at roughly 20 times the potency of fentanyl, which is itself about 50 times stronger than heroin. The DEA first identified ISO in the U.S. illicit market in 2019; by 2026 it had reached San Francisco’s Tenderloin.

What makes nitazenes uniquely dangerous is invisibility. They are not chemically related to fentanyl, so the fentanyl test strips San Francisco distributes by the hundreds of thousands will not flag them. A user who tests negative for fentanyl can still be holding a drug 20 times more lethal — and the overdose, when it comes, can require multiple doses of naloxone, if naloxone works at all.

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Fentanyl arrived in San Francisco in the summer of 2015. By the end of 2025 it had killed more than 3,000 people in the city. The death toll peaked in 2023 at 810 fatal overdoses — the deadliest year on record — before falling to 621 in 2025. City officials touted the 23% decline as progress; the same data showed San Francisco County still ranked second in the nation for fatal-overdose rate, behind only Baltimore.

The improvement, such as it was, came as the supply was already shifting underneath the city’s feet. In late 2025, the forensic alert network logged a cluster of deaths tied to cychlorphine — an “orphine” analogue roughly 10 times stronger than fentanyl. Months later it surfaced in San Francisco blended with ISO. A falling death count is cold comfort when each remaining dose is far more likely to kill.

San Francisco bet heavily on naloxone. The city pushed citywide distribution from 47,000 doses to more than 158,000 in a single fiscal year — a genuine achievement against ordinary fentanyl. But the new compounds are built to defeat exactly that defense. The DEA warns that for carfentanil — the 100-times-fentanyl tranquilizer the agency’s San Francisco division flagged in a 5,000-pill seizure in September 2025 — “multiple, high doses of naloxone may be required, and even then, effective reversal of an overdose is not guaranteed.”

“Multiple, high doses of naloxone may be required, and even then, effective reversal of an overdose is not guaranteed.”

U.S. Drug Enforcement Administration · Carfentanil public warning

Nitazenes carry the same warning label. Anti-drug activist Jacqui Berlinn, who founded Mothers Against Drug Deaths after her own son’s years on the street, put it plainly: the new poisons are “taking the addicts unaware.” A harm-reduction strategy that assumes the rescue drug will work is no strategy at all when the supply is engineered to outlast it.

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For most of the fentanyl decade, San Francisco’s governing philosophy was harm reduction: distribute clean supplies and naloxone, minimize arrests, meet users where they are. The state poured money behind it — California allocated $61,000,000 to sustain and expand its Harm Reduction Initiative, funding fentanyl test strips, naloxone delivery, and overdose-response training. San Francisco itself drew more than $11,000,000 in federal overdose-prevention support.

Mayor Daniel Lurie (D-SF), who took office in January 2025, has tried to pivot. He signed “Recovery First” legislation, halted the no-strings distribution of drug supplies, and opened a RESET Center as an alternative to jail or the ER for street users. But the pivot ran straight into a supply that had already escalated past the tools the city stocked. District Attorney Brooke Jenkins (D) blames lenient local judges: of six narcotics traffickers arrested in a recent Tenderloin sweep, she said, five were released by the court.

The chemistry keeps moving faster than the policy. As Chinese government controls clamped down on nitazene precursors, traffickers pivoted to fresh analogues — cychlorphine and the broader “orphine” class the DEA has begun logging in its forensic system. DEA official Jonathan Pullen described the treadmill bluntly: “The drug market is evolving. As we pressure one substance, they pivot to another. There’s always something new, and it’s usually more dangerous.”

Carfentanil makes the same point in numbers. The DEA identified it in roughly 1,400 seizures in 2025, up from 145 in 2023 and just 54 in 2022. Each new compound is pressed into counterfeit pills — fake M30s built to look like prescription oxycodone — so the buyer believes they are getting a known quantity. They are not.

The honest reckoning is that San Francisco’s entire response apparatus was calibrated to a single molecule. Test strips detect fentanyl. Naloxone reverses fentanyl. Public messaging warned about fentanyl. Nitazenes, cychlorphine, and carfentanil each break at least one of those assumptions — and they are arriving together, blended, in pills users believe are something tamer.

Closing the gap means tools the city has been slow to deploy at scale: nitazene-specific test strips, drug-checking spectrometers in the field, real-time supply surveillance from the medical examiner, and the enforcement to choke off the counterfeit-pill pipeline. Lurie’s enforcement turn is a start. Whether it outpaces a drug supply that mutates every season is the question the next overdose count will answer.

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