Boulder County Public Health is warning residents of potentially deadly counterfeit medications found to contain fentanyl and other dangerous additives.

Illegally manufactured pills made to look like the oxycodone and Xanax were recently found in Boulder containing fentanyl, the department said Friday.

Fentanyl is a synthetic opioid around 50 times more potent than heroin and 100 times more than morphine.

Of the counterfeit pills containing fentanyl, one in four had a potentially lethal dose, the department said.

“These pills are designed to look like legitimate pills but are made with ingredients that may contain fentanyl or other dangerous additives that can cause serious medical complications or can lead to death,” the department said.

For anyone who might use these counterfeit pills, the department suggests carrying Narcan, a prescription medicine used to treat opioid overdoses.

Narcan is available for free at Boulder County Public Health and the University of Colorado Boulder’s Wardenburg Health Center, and can be found at most pharmacies.

The department also recommends testing drugs using fentanyl strips, starting with small doses, never using drugs alone and calling 911 if someone has overdosed.