Doctors in Philadelphia are reporting cases of heroin overdoses they suspect involve the synthetic opioid W-18. The drug, which can be added to heroin without the user’s knowledge, may be too strong for the overdose antidote naloxone to reverse, according to The Philadelphia Inquirer.

Anita Gupta, an anesthesiologist, pharmacist and pain specialist at Drexel University College of Medicine, said months ago she started seeing overdose patients who did not respond as they should have to naloxone. “The symptoms were worse than we were used to seeing,” she said. “We were getting patients with symptoms of near-death, and often required multiple doses of the antidote naloxone.”

“We’re seeing more unexpected overdoses in patients who were chronic, stable users, suggesting there’s a contaminant in the heroin they were using,” said Jeanmarie Perrone, Director of Medical Toxicology at the Hospital of the University of Pennsylvania. She noted most hospital laboratories are not equipped to spot W-18.

According to the article, the Drug Enforcement Administration recently circulated a bulletin warning W-18 is said to increase the strength of heroin and cocaine. Law enforcement officials say they have not been able to prove that W-18 has killed anyone in Philadelphia.

Last month, police in Edmonton, Canada reported the seizure of a large amount of W-18, which they say is 100 times more powerful than fentanyl. The Edmonton police seized 4 kilograms, enough to make hundreds of millions of pills. The drug is likely being produced in China.

In March, the Sun Sentinel reported that a drug dealer in Florida who was convicted of importing fentanyl also had 2.5 pounds of W-18.

According to CBC News, W-18 and a number of similar compounds were invented by researchers at the University of Alberta in the 1980s as potential pain relievers. The drugs were patented in 1984, but had no known use outside of scientific research.