San Francisco is grappling with the emergence of a potent new drug, ISO, which is reportedly twenty times stronger than the already lethal fentanyl. Authorities have issued warnings as the drug’s impact becomes increasingly evident on the city streets.
Isotonitazene, commonly referred to as ISO among users, first resulted in a fatality in San Francisco in April, according to a health alert released by the San Francisco Health Department. This counterfeit opioid has raised significant concerns in the city already struggling with a drug crisis.
Health officials reported that ISO was mixed with another synthetic opioid, cychlorphine, in the overdose case reported on April 23. Given its low cost and extreme potency compared to fentanyl, ISO poses a severe threat to public health.
The department’s alert emphasized the danger of ISO, stating that even a single dose could be fatal. The alert also highlighted that these synthetic opioids do not appear on standard drug tests for fentanyl, making them particularly hazardous as they remain undetected.
A disturbing video circulating online captures the grim reality of ISO’s impact, showing numerous individuals in a dazed state in San Francisco’s infamous 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.
