Public health officials warned Monday against fake opioid pills that contained fentanyl.

The pills, which looked like oxycodone, tested positive for the synthetic opioid drug late last week, according to Seattle & King County Public Health.

“Markings on these pills looked like those typically found on some oxycodone pills, so a user may think they are taking oxycodone when in fact, the pills are counterfeit and contain fentanyl,” said Dr. Jeff Duchin, a health officer for Seattle & King County Public Health.

Health officials said in a release they were worried there is a potential for an increase of fentanyl in western Washington, because other areas of the country have seen the drug cause increased overdose deaths.

Fentanyl is 100 times more powerful than other opioids and can be found in pills, powder, or heroin. Officials say you cannot smell or taste fentanyl, and you can’t tell if it’s in a batch just by looking at it.

The amount of fentanyl in a pill is not always consistent and can vary within a batch, which could lead to overdose.