The Drug Enforcement Administration is warning the public that counterfeit prescription pills that include potentially lethal doses of fentanyl, made by Mexican drug cartels, are making their way into the United States.

A DEA bulletin issued Monday says these counterfeit pills are being mass produced for distribution throughout North America. A sampling of tablets seized in the first three months of 2019 found 27% contained potentially deadly doses of fentanyl.

Fentanyl is a powerful synthetic opioid that can be deadly even in small amounts. The DEA says a lethal dose of fentanyl is estimated to be about two milligrams, but that can change based on a person's tolerance, body size, previous drug usage and other factors.

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According to the Centers for Disease Control and Prevention, fentanyl is approved for treating severe pain, typically advanced cancer pain. It is 50-to-100 times more potent than morphine. But most cases of overdose and death from fentanyl are due to versions of the drug that are illegally manufactured and sold. 

Overdose deaths involving synthetic opioids other than methadone increased almost 47% from 2016 to 2017, the CDC reports. 

The DEA bulletin did not explain how one of these counterfeit pills could be differentiated from regular pills.