The Food and Drug Administration targeted more than 500 websites it said were illegally selling unapproved versions of prescription medications, including opioids, antibiotics and injectable epinephrine products, the agency said Monday.

The action was part of a global operation called Pangea X, led by the international police organization Interpol. That group said the international enforcement effort, designed each year to identify the makers and distributors of illegal, counterfeit and substandard medical products on the Internet, occurred Sept. 12 to 19.

This year's crackdown involved police, customs and regulatory officials from 123 countries and resulted in the seizure of millions of doses of dietary supplements, pain reduction pills, epilepsy medication, erectile dysfunction pills and antipsychotic medication, Interpol said.

The FDA, for its part, focused on websites that illegally sell opioids online and ship them through the U.S. postal system to U.S. consumers. The agency said it sent 13 warning letters to the operators of more than 400 websites and seized nearly 100 domain names, such as buyhydrocodoneonline.com and  buyklonopin.com.

“These rogue online pharmacies are often run by sophisticated criminal networks that knowingly and unlawfully distribute illicit drugs, including counterfeit medicines and controlled substances,” FDA Commissioner Scott Gottlieb said in a statement. “The ease with which consumers can purchase opioid products online is especially concerning to me, given the immense public health crisis of addiction facing our country.”

Gottlieb, who has said that curbing the opioid epidemic is his top priority, added that the FDA action was part of a broader initiative to slow the flow of illegal drugs through international mail facilities located in the United States. The agency recently increased the numbers of inspectors to step up surveillance at the facilities.

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