NEW YORK (CNN/Money) - Pssst! Want some Viagra? How about Xanax? Prozac?

Most everyone who surfs the Web had seen sites advertising prescription drugs. After all, online and mail order sales represent about 25% of the $200 billion drug industry in the U.S., according to Carmen Catizone, executive director of the National Association of Boards of Pharmacy.

But many of the sites being advertised are illegal. Catizone estimates that the illicit online pharmaceutical industry could top $1 billion.

In their research of illegal sites, the NABP and the Food and Drug Administration have found an open marketplace for illicit drugs. By all accounts, illegal drugs are easy to purchase online, with no prescription required.

"You can buy anything you want," said Catizone, who pointed to painkillers and weight loss drugs as popular Internet contraband. "We've ordered steroids on eBay, where they disguise them as books. The only thing we haven't been able to get is Oxycontin, I think because there's so much notoriety around it."

Some of the illegal online drug retailers aggressively pursued investigators, calling them during the holidays with offers of weight loss drugs, said Catizone. "The brazenness and the disregard is amazing," he said.

While the illegal industry is not big enough to pose a significant threat to legitimate online pharmacies, it does pose a health threat to American consumers, according to the FDA, which has devoted much of its web site to health warnings against illicit online drug shopping.

"We don't know where they're being made, how they're being made, where they're being shipped, how they're being shipped," said Tom McGinnis, director of pharmacy affairs for the FDA. "It puts the consumer in a buyer-beware type of situation."

But many consumers don't care.

"They buy drugs from countries where they wouldn't drink the water," said Catizone, referring to countries in the Caribbean, Pacific Rim and former Soviet states. "What's driving this whole issue is price. People are ignoring the warnings and the legality because they're looking for the best price."

Some of the drugs are manufactured in foreign countries, legally or illegally, and then shipped into the U.S., where they have not been approved by the FDA. Other drugs are manufactured in the U.S. for foreign markets, exported to those countries, and then imported illegally back to their country of origin.

Canada has garnered a reputation as a source for cheap pharmaceuticals and the maple leaf is often touted on web sites that claim to be based there. The FDA recently launched a study to determine how many drug sites claim Canadian citizenship.

Cyveillance, Inc., an online risk monitoring company based in Arlington, Va., conducted the study for the FDA and discovered about 11,000 sites claiming to be Canadian-based drug retailers. However, only 1,009 of those sites actually sold pharmaceuticals, and only 214 had registrants or owners with Canadian addresses, according to Cyveillance. The other 10,000 sites had no discernible addresses and no means of purchasing drugs, but plenty of Canadian flag icons, said Todd Bransford, vice president of marketing for Cyveillance.

"People trust the Canadian government," said Bransford. "Most people perceive the Canadian government does a good job of vetting the drugs for safety. Since you have this demand, you have fraudsters who use this demand against the consumer."

So how does the consumer tell the illegal sites from the legal ones? The NABP has published a list of 14 online pharmacies that it considers legitimate on its website www.nabp.net. These companies have been registered with state pharmacy boards in the U.S. There may be other legal sites that are not included on NABP's list. The list includes Drugstore.com, an online retailer based in Bellevue, Wash. that made $156 million from drug sales in 2004, nearly half its total revenue.

Identifying a site's legitimacy is not so hard, said Greg French, spokesman for Drugstore.com (down $0.27 to $4.29, Research). Ignore the little Canadian flags and see if the site has a direct link to the NABP's web page, the badge of legitimacy, French said.

Another big hint: real sites require a prescription.

"Be wary of any site that does not require a valid prescription based on a personal visit with a physician," said French. "Some of these sites will actually provide medication with an online questionnaire or sometime no questionnaire at all."

Also, legal sites will have easy-to-find mailing addresses with phone numbers connecting the caller to the business, said French. "If you get a voice mail that sounds like a personal answering machine, that's a little cause for concern," said French.

French also warned against prices that are "way too good to be true" and spam mail advertising the powers of Viagra because "legitimate pharmacies don't spam."

The illegal activity has created an unsavory reputation that has dogged legitimate online pharmacies, said French, but he hopes that will change with name recognition and trust-building.

"When there are stories about people being ripped off by these players, it doesn't do us any good," said French. "I think as consumers become more savvy in the space and know what to look for, there will be less resistance."

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