Americans who turn to online pharmacies to find cheaper versions of expensive prescription medications, especially opioids, may instead be buying themselves dangerous drugs that could trigger an overdose, U.S. health officials warned Wednesday.

The warning, issued by the U.S. Centers for Disease Control and Prevention, follows a federal indictment announced last week against 18 individuals who are accused of running a massive illegal online pharmacy operation.

"The victims of the scheme include people from all walks of life — people of all ages and occupations — from all 50 states, the U.S. Virgin Islands, Puerto Rico, Germany and Slovenia," the U.S. Department of Justice said in a statement announcing the indictment. "At least nine victims who purchased counterfeit pills from the defendants died of narcotics poisoning, including a 45-year-old army veteran who thought she was purchasing real oxycodone."

In its warning, the CDC noted that the counterfeit pills frequently contained fentanyl, a synthetic opioid that is up to 50 times stronger than heroin and 100 times stronger than morphine, and methamphetamine.

The problem is pervasive: According to the National Association of Boards of Pharmacy, almost 95% of online pharmacies operate illegally and 24% of Americans who have used an online pharmacy have been exposed to harmful, counterfeit or substandard medications.

In light of that, people should only take medications prescribed by a licensed healthcare provider and dispensed by a licensed pharmacy, the CDC said.

What should you do if you have bought what you believe to be prescription medications from an online pharmacy?

The CDC recommends:

• Check to see if the pharmacy is state-licensed. If you got the medications from a pharmacy that is not licensed, do not consume the medication

• If you think you have counterfeit drugs, dispose of them safely

• Always have naloxone, an opioid overdose reversal drug, with you. It should be given immediately to anyone who shows signs of an opioid overdose. Always have someone nearby whenever you take an unknown medication, in case of emergency.

• Call 911 and seek medical attention immediately if you think you have been poisoned

• For questions about an unknown substance, contact Poison Control.