In some cases, online pharmacies have been selling drugs for up to 90 percent off retail -- but at these prices, the pills may offer much less than they promise. "When a pill is selling for 10 percent of the retail price, you know it didn't come through a standard distribution network," says Fred Felman, chief marketing director of MarkMonitor. "It's probably fake, expired, or an offshore generic that wasn't inspected by the Food and Drug Administration."

In an investigation of mail-order oseltamavir drugs, the FDA noted that the pills often took too long to arrive, had inconsistent dosages, and sometimes didn't even contain oseltamavir. One shipment contained pills composed of acetaminophen and talcum powder.

Not all off-brand drugs are placebos: many disreputable drug manufacturers also purchase bulk quantities of active pharmaceutical ingredients and produce their own pills. While these capsules often contain the active ingredient that they claim, their production facilities are not as controlled as those of mainstream pharmaceutical companies; some, Felman says, have "work benches covered in dust and dirt, and use cement mixers to coat and polish their tablets."

With fear over swine flu reaching a fever pitch and health insurance more pricey than ever, online pharmacies are huge. Business-to-business sales of active pharmaceutical ingredients have risen 23 percent this year and, according to MarkMonitor estimates, online pharmacies are generating $10.7 billion in revenue, up from roughly $4 billion in 2007. While 49 percent of these pharmacies were based in China, the operations are global: a North American phone number connects to a Russian-accented operator selling drugs shipped from India and billing the buyer's credit card through Israel.

This isn't to say that all online pharmacies are scams. One site, LegitScript, certifies online pharmacies' legitimacy. Particularly vigilant users can visit the Web sites of the Canadian International Pharmacy Association or the Verified Internet Pharmacy Practice Site to ensure that a particular pharmacy is accredited.