WEDNESDAY, Feb. 7 (HealthDay News) -- The first non-prescription drug to treat obesity in American adults was approved Wednesday by the U.S. Food and Drug Administration.

The drug, called alli (orlistat), is designed to be used only in tandem with a reduced-calorie, low-fat diet by overweight adults 18 and older. According to manufacturer GlaxoSmithKline, the drug helps people lose 50 percent more weight than dieting alone, should cost consumers $12 to $25 a week and is expected to be available by this summer.

"This is the only FDA-approved, over-the-counter weight loss drug product," Dr. Charles J. Ganley, the FDA's director of the Division of Over-The-Counter Drug Products, said during a teleconference. "There are some products, primarily dietary supplements, that make weight-loss claims and those are not FDA-approved, although they are permitted to make these claims."

The drug will be available in 60 milligram capsules, to be taken three times a day with meals that contain fat. The company said the drug works by "blocking about 25 percent of the fat in food a person eats. Because of the way it works, alli must be used in conjunction with a reduced-calorie, low-fat diet containing about 15 grams of fat per meal."

Eating a meal with too much fat while taking the drug can result in bowel changes such as loose stools, according to the FDA. These side effects typically occur in the first weeks of treatment, they aren't harmful, and can be managed by following the recommended diet of about 15 grams of fat per meal, GlaxoSmithKline said.

It's also recommended that users take a multivitamin once a day, at bedtime, because the drug can interfere with the absorption of some vitamins, GlaxoSmithKline said.

People who have had an organ transplant shouldn't take the drug. And anyone taking blood-thinning medicines or being treated for diabetes or thyroid disease should consult a physician before using the drug, the FDA said.

"This drug is only going to be effective if it's used along with a weight-loss program," Ganley said. "That means a reduced fat diet, decreased calories and an exercise program."

"If someone uses the drug without a weight-loss program, it's not going to be very effective," he added.

A higher dose of orlistat (120 mg capsules) has been marketed as the prescription drug Xenical in the United States since 1999.

While the company claims Xenical's safety has been demonstrated by nine years of worldwide use in 146 countries, the consumer-advocacy group Public Citizen last year petitioned the FDA to remove Xenical from the U.S. market.

Public Citizen contended that the higher-dose drug may increase the risk of aberrant crypt foci, which are widely believed to be a precursor to colon cancer.

Late Wednesday, the advocacy group issued a statement criticizing the FDA's approval of the new over-the-counter drug.

"At a time when colon cancer is a leading cause of death and disease in the United States, the Food and Drug Administration's decision to approve, for over-the-counter use, a diet drug that clearly causes pre-cancerous lesions of the colon is the height of recklessness and shows a profound lack of concern for the public's health," Dr. Sidney M. Wolfe, director of Public Citizen's Health Research Group, said in the statement.

One obesity expert isn't sure the new OTC drug will be effective for most people.

"The drug is probably safe," said Dr. Raj Padwal, an assistant professor of general internal medicine at the University of Alberta. "However, I'm not sure the half-strength dosage will have much effect."

Full-strength dosage reduces weight by less than 3 kilograms (6.6 pounds), on average, Padwal said.

"People may only lose 1 to 2 kilograms (2.2 to 4.4 pounds) on this half-strength dose. Whether that is worthwhile is questionable. The occasional patient may benefit but many patients may not. For those patients who need extra incentive to adhere to a low-fat diet, the drug may help," he said.

The FDA's approval of the first over-the-counter drug for weight loss comes as the United States and other western nations are struggling with an unprecedented obesity epidemic.

According to the U.S. National Center for Health Statistics, 30 percent of American adults 20 years of age and older -- more than 60 million people -- are obese. And another 36 percent are considered overweight.

More information

For more information on orlistat, visit the U.S. National Library of Medicine.