Approving a new weight-loss drug is not something the Food and Drug Administration takes lightly—with good reason. In 1997, the appetite suppressants fen-phen (a combination of the drugs fenfluramine and phentermine) and Redux (dexfenfluramine) were withdrawn after it was suspected that they caused heart-valve defects, leaving the millions who relied on them hungry for a new fix. Now comes orlistat (brand name Xenical), a dietary fat-blocker that earned FDA approval in April. Available in pharmacies—and over the Internet—it is already generating a Viagra-like frenzy.

There is a difference, though, says Dr. Eric Colman, 37, the FDA scientist in charge of reviewing the drug. With Xenical, "you may spend more time in the bathroom than in the bedroom," he says, referring to the drug's common and unpleasant side effects. Other scientists, among them Dr. Jules Hirsch, professor emeritus at Rockefeller University in New York City, have been highly critical of Xenical, deeming it virtually ineffective and hardly worth the trouble.

To be sure, Xenical is no wonder drug: In a yearlong clinical trial, obese patients on a reduced-calorie diet who did moderate exercise and took the drug lost an average of 13 pounds and saw modest improvements in blood-pressure and cholesterol levels. Even the company that markets it downplays its powers. "This is not a quick fix," says Dr. Russell Ellison, a vice president at Hoffmann-La Roche. "Losing weight involves work."

That's not news to the 97 million Americans who are overweight. "Diet and exercise have been around forever, " says Colman, a graduate of Philadelphia's Temple University School of Medicine who specializes in metabolism. "Have they worked? No. That's why people are dying for a drug." At the FDA office in Rockville, Md., Colman, who lives with his wife, Patricia O'Maille, a painter, and their 3-year-old son in Kensington, Md., spoke with contributor Macon Morehouse about Xenical's pros and cons.

How does Xenical work?

It's a chemical that blocks the enzyme that breaks down dietary fat in the intestine. If fat is not broken down, it can't be absorbed. Xenical blocks roughly 30 percent of dietary fat. Undigested fat is excreted.

How does it differ from Redux and fen-phen?

Those drugs are appetite suppressants that work on certain brain chemicals to send your brain the message that you're not hungry. Very little of Xenical is absorbed into the bloodstream, which probably makes it safer than others.

Who should take Xenical?

People who are obese; that is, with a body mass index [your weight in kilograms divided by the square of your height in meters] of 30 or greater. For example, a woman 5'5" who weighs more than 180 pounds would qualify.

Who should avoid Xenical?

People who have chronic digestive problems. Also, people with gall bladder problems, pregnant women or those about to become pregnant, and women who are breast-feeding.

What about people who are just 10 or 20 pounds overweight?

I'm sure many of them will take it, but I wouldn't recommend it. For one thing, it's expensive [between $4 and $5 a day]. They could just as easily cut back on the fat and calories in their diets and get the same results without the side effects.

What are the side effects?

There's an increased urge to go to the bathroom; fatty or oily stools; fecal incontinence. These side effects aren't dangerous, but if you eat a huge fatty meal and then take Xenical, you'll probably get more side effects. So people who take it may learn to avoid fatty foods.

Can children take it?

Physicians can freely prescribe this for anyone they want, but the safety and efficacy have not been established in children under 16.

Thousands of people have already purchased Xenical over the Internet. Are there dangers in using it without a doctor's supervision?

Yes. In theory it's safer to have a physician monitoring a patient's response to any prescription drug.

What is the difference between Xenical and the Olean in fat-free chips?

Olean is not a drug. It is a fat substitute that is not absorbed by the body. But it can have similar side effects as Xenical.

How long should Xenical be taken?

That's up to physicians. The studies followed patients for up to two years. From what we've seen, the benefits over that time outweigh the drawbacks. But as we learned with fen-phen and Redux, there can be side effects that are not picked up during the trials. If someone takes Xenical for five years, I can't tell you something weird won't happen.

Is Xenical a substitute for exercise and diet?

It's recommended that people lower their calorie intake and exercise more. But I suspect a lot of people won't do that, and those people won't lose as much weight.