Pfizer Inc., in a move to thwart counterfeit Viagra, on Friday said it has included special radio frequency identification tags on all packages of its anti-impotence pill to verify they are the authentic Pfizer product.

The world’s biggest drugmaker said the new technology, which is difficult and costly to duplicate, would create barriers “for criminals who might attempt to counterfeit our products.”

Pfizer has previously also opposed illegal imports of authentic Viagra into the United States, where they can be sold at steeply discounted rates, but said the new technology is not designed to block such imports.

“We honestly have not looked at this from an anti-importation perspective,” said Pfizer spokesman Bryant Haskins, whose company has vigorously opposed U.S. legislation that would legalize importation of prescription drugs.

Haskins said the tiny tags are small computer chips that have been affixed to the underside of labels on each bottle of Viagra, as well as on cases and pallets of the drug. The invisible tags relay an electronic code that verifies the product is bona fide and authorized Viagra.

“Pharmacists and wholesalers use specially designed electronic scanners that communicate the code over the Internet to a secure Pfizer Web site” for verification purposes, the New York-based company said.

Haskins said Pfizer is the first large drugmaker to put the radio-tag technology to use but said privately held Purdue Pharma LP already uses such tags to monitor shipments of its widely-abused OxyContin pain drug.

Although the technology can physically scan packages of Viagra and thereby authenticate them, Pfizer said it cannot yet “track and trace” the whereabouts of packages as they are distributed across the United States.

Tracking and tracing would require all parts of the distribution system to invest in compatible technology and agree to use it to share information, Pfizer said.

The company said it plans to further explore tracking and tracing abilities of the technology and how the radio tag technology might also help thwart counterfeit versions of other Pfizer drugs.

Viagra had global third-quarter sales of $386 million, almost half of which were in the United States. Although it remains one of the company’s biggest products, Viagra revenue has slipped amid competition from the longer-acting Cialis treatment sold by Eli Lilly and Co. and Icos Corp.

Pfizer last February filed lawsuits against a number of Web site operators that it alleged have sold illegal versions of Viagra. At the time, Pfizer said it did not know how much the company was losing in Viagra revenue as a result of illegal Internet sales.