The number of people that police in St. Louis have been able to save using the heroin overdose antidote naloxone, or Narcan, has declined by approximately 30 percent this year, compared to last year, according to the Associated Press.

The St. Louis County Medical Examiner’s Office reported that nearly 90 percent of 121 overdose deaths through July of this year involved the drug fentanyl. Sometimes mixed with or sold as heroin, fentanyl is a powerful opiate that is considered stronger than heroin, making reversing an overdose from fentanyl more difficult.

“The toxicity level of fentanyl is so potent, it might not be reversible,” said Spring Schmidt, director of health promotion and public health research for St. Louis County. “The potential for death is faster, and that impacts our ability to reverse an overdose.”

Health officials noted that fentanyl overdoses may require more than one dose of Narcan to successfully revive patients. St. Louis County officer Benjamin Granda said that this year, officers gave three doses to an overdose victim who died.

“Speaking in general terms, the more potent the drug, the quicker the individual could overdose,” Granda said. “It is entirely possible that these individuals are just not being found fast enough.”