Pfizer Inc., the world’s largest drug maker, has started offering its impotence drug Viagra through a company-sponsored website to combat counterfeit versions that are sold online.

Men with prescriptions for the erectile-dysfunction medicine can order the drug at Viagra.com. The prescriptions will be verified and completed by Woonsocket, Rhode Island-based CVS Caremark Corp., the largest U.S. provider of prescription drugs.

“There are almost 24 million searches a year for Viagra online,” said Victor Clavelli, senior director in Pfizer’s primary care unit, said in a company statement. By helping men obtain Viagra through the official website, the company seeks to “rein in the distribution” of fake versions of the drug.

Pfizer in 2011 conducted a chemical analysis of pills purchased from 22 websites that ranked high in an online search for the phrase “buy Viagra.” The analysis found that roughly 80 per cent of the pills claimed to be Viagra were counterfeit and didn’t contain full doses of the active ingredient as advertised, the New York-based company said in the statement.

Sales of Viagra were $2.05 billion in 2012, Pfizer reported in a regulatory filing.

The website “is providing a resource for patients to be able to get their prescription in a safe and legitimate way,” Jennifer Kokell, a spokeswoman for Pfizer, said in a telephone interview. “We expect to learn from this.”

Pfizer shares rose less than 1 per cent to $28.94 at 11:48 a.m. ET.