In four months on the market, more than 2 million starter packs of over-the-counter diet pill alli (low-dose orlistat) have been purchased by U.S. dieters, according to GlaxoSmithKline Consumer Healthcare.

As the only FDA-approved weight-loss product available without a prescription, alli -- a half-strength version of prescription diet drug Xenical -- has gotten off to a surprisingly strong start, aided in part by the June collapse of efforts to win FDA approval of the highly anticipated prescription diet drug Acomplia.

The alli capsule works by blocking the absorption of about 25 percent of the fat in the foods people eat, with the undigested fat then removed from the body in bowel movements.

Few observers expected alli sales to reach this level, given that it results in relatively little weight loss and if a person consumes too much fat, produces a variety of unpleasant side-effects.

But the absence of any other new diet drug on the market this year clearly has led a number of overweight and obese Americans to give alli a try.

Glaxo said that in addition to its retail success, alli has garnered intense consumer interest, with approximately 5.4 million unique visitors logging on to the official alli websites, myalli.com and the Spanish-language mialli.com.

Glaxo said more than 200,000 people also have enrolled in alli's online behavioral support program, and consumers have also posted more than 125,000 messages on the official alli message boards.