The streets of San Francisco are awash with a new drug called ISO that’s twenty times more potent than deadly fentanyl – and officials are sounding the alarm.
The killer counterfeit opioid isotonitazene – known on the streets as ISO – claimed its first life in the drug-ravaged city in April, San Francisco Health Department officials said in an alert issued that month.
San Francisco health officials said ISO was blended with another synthetic opioid called cychlorphine in the overdose death announced April 23. ISO is cheap and many times more powerful than fentanyl – making for a potential crisis.
“Even a single pill can contain a lethal dose,” the officials said in their alert, which noted that the two synthetic opioids would not show up in tests that indicate the presence of fentanyl, making them even more dangerous because they’re undetectable.
A shocking video posted online showed throngs of stupefied users reeling from the effects of the deadly drug in the city’s notorious Tenderloin district.
In the video created by Manhattan Institute writer Christopher Rufo, addicts are seen stooping over from the effects of the powerful drug and even lying corpselike on the sidewalk.
Rufo said a dealer told him ISO is becoming more popular on the streets.
“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,” said Rufo. “It’s devastating California cities.”
According to the Center for Forensic Science Research and Education, isotonitazene, which is also known as Nitazine, is roughly 20 times more potent than fentanyl. It may be smoked, used intravenously or taken in a pill.
Officials with the federal Drug Enforcement Administration last month issued a new warning about ISO and other synthetic drugs that the DEA says may be mixed with fentanyl.
“These emerging synthetic drugs can be significantly more powerful than fentanyl and greatly increase the risk of suffering a fatal overdose,” the advisory warned, noting that users who OD on the drugs may not be responsive to the overdose-reversal drug naloxone.
According to the DEA, ISO initially emerged in U.S. illicit drug markets in 2019, when it was first identified in seizures of illegal drugs by law enforcement. ISO is not approved for medical use and has no industrial use.
ISO is a “serious concern to the public safety,” the DEA said in another bulletin issued on the drug in December 2025.
The DEA has counted more than 900 reports of isotonitazene since it was initially identified by the agency in 2019, including 42 records in 2019, a peak of 358 reports in 2021, and 43 reports in 2024, the most recent year for which the data was available.
Law enforcement officials in Los Angeles said ISO has not yet become a threat in California’s biggest city, but one police source said that “bathtub pharmacists” there are increasingly mixing fentanyl with a variety of other drugs, including animal tranquilizers.
Los Angeles health officials blame fentanyl for just over half of all drug and alcohol deaths in the city. Police have recently mounted a push to cleanse MacArthur Park in downtown LA of the crowds of users who purchase and consume drugs there.
Jacqui Berlinn, who cofounded the San Francisco-based activist group Mothers Against Drug Addiction and Deaths, said San Francisco police ought to be doing more to clean up the streets of that city.
Berlinn said the rise of ISO is particularly threatening because the drug is so powerful and it can’t be detected by fentanyl test kits.
“It’s incredibly scary because it’s taking the addicts unaware,” said Berlinn.
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