China has banned a class of potent synthetic opioids that includes cychlorphine, a drug linked to the fatal overdose of a 16-year-old San Francisco boy and flagged by local health workers and police in recent months. The ban could limit cychlorphine’s spread into the U.S. drug supply, experts hope. But the drug presents a difficult public-health challenge: warnings about a substance reported to be as much as 10 times more potent than fentanyl could also draw interest from people who say fentanyl no longer works for them. Cychlorphine is part of a group of synthetic opioids called orphines. Unlike fentanyl, it has a distinct chemical structure. Officials caution that an overdose may require several doses of naloxone, the medication commonly sold as Narcan, to reverse. The drug was first detected in the U.S. supply in 2024. The next year, it was linked to a fatal overdose cluster in Tennessee and found in multiple parts of the country. By the end of 2025, cychlorphine had appeared in more than 100 forensic drug reports nationwide, according to federal officials. Toxicologists do not know how many people knowingly took it and how many were exposed without realizing it. In April, the San Francisco teenager died after taking cychlorphine in a pill he believed was Xanax. In September, police announced the arrests of four people accused of involvement in selling the drug in the city. Cychlorphine belongs to a class first synthesized by Belgian chemist Paul Janssen in the 1960s. Its appearance in U.S. drug reports, the San Francisco death and the arrests underscore the local stakes as officials try to alert people to a dangerous drug without inadvertently promoting it.