Law enforcement officials say they are seeing increasing cases of the potent opioid fentanyl being sold as other painkillers, such as oxycodone or Percocet.

In Tennessee, officials say there have been two dozen cases in recent months of pills marked as oxycodone or Percocet that turned out to include fentanyl, according to the Associated Press.

Fentanyl is 25 to 40 times more powerful than heroin, the article notes. It is used for treatment of chronic pain in end-stage cancer patients.

San Francisco’s health department said several overdoses last summer were due to fentanyl that looked like Xanax. Canada has issued warnings about fentanyl pills that look like oxycodone.

Federal agents arrested a man in suburban Cleveland in February after seizing more than 900 pills containing fentanyl that were marked as oxycodone tablets. Carole Rendon, acting U.S. Attorney in Cleveland, explained fentanyl is cheap to make, so dealers sell them as other drugs, so they can charge more money.

Dr. Thomas Gilson, Medical Examiner in Cuyahoga County, Ohio, said fentanyl pills disguised as other painkillers were likely to blame for some of the county’s 19 fentanyl-related overdose deaths in January. “People might otherwise say, ‘I know I can abuse this much of oxycodone,’ and they may be in for a really, really bad surprise when they find out that’s fentanyl and not oxycodone,” he said.

On its website, the Drug Enforcement Administration (DEA) notes that fentanyl “is a very dangerous substitute for heroin because it is much more potent than heroin and results in frequent overdoses that can lead to respiratory depression and death.”

“The fact that fentanyl has been found in this form should hopefully make people nervous that do abuse these types of opiate pills, that they could be getting their hands on something even more lethal,” said DEA spokesman Rich Isaacson.