The online Viagra sales are Pfizer's latest effort to combat a problem that has grown with the popularity of the Internet.In recent years, Americans have become more comfortable with online shopping, with many even buying prescription drugs online. That's particularly true for those who don't have insurance, are bargain hunters or want to keep their medicine purchases private. Few realize that the vast majority of online pharmacies don't follow the rules, industry experts say. The Internet is filled with illegitimate, professional-looking sites that run 24-hour call centers and lure customers with spam emails. A January study by the National Association of Boards of Pharmacy, which accredits online pharmacies, found that only 257 of 10,275 online pharmacy sites it examined appeared legitimate. That means the sites require a doctor's prescription, are based in the U.S., only sell FDA-approved drugs and have a secure server site so customers don't have their credit card and identity information stolen. Experts say the fake drugs such websites sell can be dangerous. That's because they don't include the right amount of the active ingredient in the medicine, if any, or they contain toxic substances such as heavy metals, lead paint and printer ink. They're generally made in filthy warehouses and garages in Asia, Eastern Europe and Latin America. Online buyers are "playing Russian roulette," says Matthew Bassiur, vice president of global security at New York-based Pfizer. "The factories are deplorable. I've seen photographs of these places," he says. "You wouldn't even want to walk in them, let alone ingest anything made in them." Pfizer is among many drugmakers that have long been aggressive in fighting counterfeiters. Pfizer conducts undercover investigations and works with authorities around the globe to combat the problem. Counterfeit versions of Viagra and dozens of other Pfizer medicines rob the company of billions in annual sales.