The Food and Drug Administration warned consumers Monday about the dangers of buying prescription drugs online after 24 related Web sites were found to be possibly distributing counterfeit medications.

In the past few months, the FDA found two sites carrying counterfeit Xenical 120 mg capsules. The weight-loss drug's active ingredient, orlistat, was missing from the pills.

The prescription drug's manufacturer, Hoffmann-La Roche Inc. tested some of the counterfeit medications and found that they contained sibutramine, the active ingredient in Meridia, a different weight-loss drug made by Abbott Laboratories, whose dosing and drug interaction differs from Xenical. Other samples contained only talc and starch.

The pills were purchased on two Web sites, and, part of 24 Web sites that appear on the Four of those Web sites have been cited in the past for selling fake Cialis and Tamiflu.

The FDA cautioned consumers about online drug deals that are probably too good to be true.

"Consumers should be wary, if there is no way to contact the Web site pharmacy by phone, if prices are dramatically lower than the competition, or if no prescription from your doctor is required," the FDA warned.

"As a result, FDA strongly cautions consumers about purchasing drugs from any of these Web sites which may be involved in the distribution of counterfeit drugs and reiterates previous public warnings about buying prescription drugs online."

The FDA issued warnings about drugs from the following 24 Web sites: