The CDC alerted public health officials and clinicians about the potential risk for drug overdose among individuals ordering counterfeit prescription medicines from online pharmacies.

The counterfeit pills sold through illegal internet-based pharmacies frequently contain fentanyl, a synthetic opioid that is the leading cause of drug overdoses in the U.S., the health agency said.

Nearly 95% of websites offering prescription drugs online operate illegally, the CDC said, citing data from the National Association of Boards of Pharmacy.

The U.S. Department of Justice pressed charges against certain illegal online pharmacies selling counterfeit prescription pills to tens of thousands of individuals in the U.S.

The CDC urged individuals to be cautious of online pharmacies that operate without a license, do not require a doctor's prescription and offer deep discounts.