They are warning consumers that the drug's high cost and limited effectiveness may not be worth its notably unpleasant side effects.
Alli (pronounced "ally"), sold by GlaxoSmithKline, is a half-dose of the prescription drug orlistat. The drug has been marketed in prescription form by Roche Holding AG under the brand name Xenical since 1999, but it never turned out to be a blockbuster.
GlaxoSmithKline, however, expects Alli ultimately to rake in at least $1.5billion in annual sales, and the company is expected to spend $150million on marketing the drug in its first year.
Television and magazine ads are running. E-tailers Amazon.com and Drugstore.com report that the drug, which has been pre-sold, already is a top seller — even at a cost of $50 to $60 for a month's supply.
Alli blocks enzymes that digest fat, preventing the body from absorbing about a quarter of the fat eaten. The undigested fat is then excreted. One study of Alli showed that dieters who took the drug along with diet and exercise over a year lost about three pounds more than people who only dieted and exercised.
The company's message is that the pills are not a "magic bullet," and officials urge users to use Alli only along with exercise and a reduced-calorie, low-fat diet.
But because Alli can interfere with vitamin absorption, it can affect how medications act in the body. The drug is not recommended for children under 18, people with kidney disease, patients on blood thinners and certain other medications, and pregnant or breast-feeding women.
Then there are the side effects, which can include oily discharge, diarrhea and uncontrollable bowel movements significant enough for the company to recommend carrying an extra pair of pants until users have acclimated to the drug. The side effects alone can force users to eat less.
"We're skeptical about something like this going over the counter," said Dr. Rohini Ashok, heading a new, doctor-supervised weight loss program. "It's not an unsafe drug, but it's not benign. The side effects are pretty gross."
As the American population grows ever fatter — about 65 percent of adults are either overweight or obese — pharmaceutical firms have tried to create a diet pill that is safe and effective, with only minimal success. A previous drug combination popularly known as fen-phen was pulled from the market in 1997 after reports of heart valve damage.
Earlier this week, an advisory committee to the U.S. Food and Drug Administration recommended against approval of the diet drug Acomplia, which has been approved in Europe but increases the risk of suicidal thoughts.
At a Weight Watchers meeting in San Jose on Thursday, Stephanie Vose, 23, said that she and her friends had tried numerous diet fads over the years, only to experience strange side effects such as shaking or bursts of energy.
"It's not like it taught you how to eat right," Vose said. "So it all comes right back."
Was Alli worth a try, given its unpleasant side effects? No way, Vose said. "That's disgusting," she said. "I would not take that."
Doctors and nutritionists wish more of their patients shared Vose's attitude and willingness to try dieting the hard way: The Weight Watchers program emphasizes portion control, exercise and slow, long-term weight loss.
"I'm sure (Alli) is not the solution to the obesity epidemic," said Christopher Gardner, a Stanford University Medical School nutrition researcher.
He painted a glum picture of how a stint with Alli might go:
The dieter starts out taking the drug, but doesn't change her diet. Gradually, she begins to lose weight, because her body isn't absorbing all of the fat she eats. Eventually, she achieves her goal and stops taking the drug. But she's in for a surprise — since she never changed her diet, her body will now absorb the fat that Alli helped reject, and her weight will balloon to where it was before.
This trap looks a lot like the ones from previous drugs, which have often worked for a small group of people and failed for everybody else, he said.
"If we had a great way for everybody to lose weight, we wouldn't need to have any of these discussions," Gardner said.
His suggestion? The usual: "Just eat less and exercise more."