TRENTON, N.J. — The Food and Drug Administration is stepping up its fight against counterfeit and other potentially harmful medicine sold over the Internet.

The agency is ordering operators of about 4,100 websites to immediately stop selling unapproved medications to U.S. consumers.

The move is part of a weeklong global crackdown by regulatory bodies and law enforcement on websites selling counterfeit, substandard and unapproved medicines, aimed at making the global medicine supply safer.

It also follows the FDA’s launch last Friday of a campaign to warn consumers that the vast majority of online pharmacies do not follow laws or pharmacy industry standards and their products could harm or even kill people.

The campaign includes a new website,, that explains the risks of fake online pharmacies and how to tell the difference between those websites and legitimate ones.

The FDA has sent warning letters to three companies behind most of the 4,100 websites it identified as illegally selling potentially dangerous, unapproved drugs. It also has seized some illegal medicines, filed civil and criminal charges against companies and individuals, and contacted Internet registrars and service providers, asking them to suspend the 4,100 websites.

“This is a first step,” said FDA spokeswoman Sarah Clark-Lynn. “The door isn’t closed on these cases.”

According to the FDA, one business, called, operates 3,710 of the targeted websites.

Another company, identified in the warning letter as Eyal Bar Oz, runs more than 200 sites. A third company, called Arkadiy Kisin/White Forest Solutions, also operates more than 200 sites.

The warning letters say that the websites have been offering unapproved drugs to U.S. consumers. The letters, sent to the companies between Sept. 18 and 21, order them to reply within 10 business days.

None of the companies responded to requests for comment from The Associated Press Friday. Clark-Lynn said she did not know whether any of the companies had responded to the FDA.

The drugs include various versions of the erectile dysfunction drugs Viagra, Levitra and Cialis, as well as an unapproved contraceptive called Norplant, an unapproved generic version of the influenza treatment Tamiflu, an unapproved antibiotic called Baycip TZ, and a drug for stomach disorders that also increases production of breast milk but is not approved because it has been linked to irregular heartbeats, cardiac arrest and sudden death.