NEW YORK — You won't lose weight in your sleep or shed pounds while eating anything you want — that's the sobering message from the maker of a weight-loss pill poised to hit shelves next month.

GlaxoSmithKline on Tuesday opened an educational exhibit in New York City to prepare the country for alli, the first over-the-counter diet pill approved by the Food and Drug Administration.

While the cautionary marketing approach may not trigger stampedes to the counter, analysts say the drug's fate hinges on the pharmaceutical giant's ability to convince people that diet pills aren't a magic bullet.

"People's hopes are ridiculously high when it comes to diet pills. That leads to disappointment and bad word of mouth," said Steven Brozak, an analyst with WBB Securities.

That's just what happened to the prescription version of the drug, Xenical by Roche Holding, which contains twice the dosage. People were let down when it failed to deliver dramatic results, and the drug never really caught on, Brozak said.

GlaxoSmithKline apparently has learned the lesson and is counting on alli to become a star money maker. The company is spending $150 million on marketing alli this year, making it one of the drug maker's biggest campaigns to date.

"We've done everything to go out of our way to be honest," said Steve Burton, vice president of the weight control division at GlaxoSmithKline Consumer Healthcare. "We're taking a very different approach than the fad diets people are constantly exposed to."

In clinical trials, the FDA says that people using alli lost an additional 2 to 3 pounds for every 5 pounds lost through diet and exercise.

When taken with meals, the drug blocks the absorption of about one-quarter of any fat consumed. That fat — about 150 to 200 calories worth — is passed out of the body, potentially resulting in loose stools.

About half of patients in trials experienced gastrointestinal side effects, including leakages and oily discharges.

GlaxoSmithKline is frank about those unpleasant effects, which it says can be controlled if the drug is used properly. The campaign stresses the importance of keeping meals under 15 grams of fat to avoid effects.

Educational materials even recommend people start the program when they have a few days off work, or to bring an extra pair of pants to the office. Experts say a failure to adequately prepare consumers about the effects contributed to Xenical's limited success.

Alli only affects the digestive system, Glaxo says, and is the only safe over-the-counter diet drug that's been shown to work.

The company estimates 5 million to 6 million Americans a year will buy the drug, translating to at least $1.5 billion a year in retail sales.

The drug will come in "starter kits" containing a food journal, a healthy-eating guide and a fat and calorie reference guide. A 60-capsule kit will cost about $50 while a 90-capsule pack will cost about $60. Recommended usage is one or two pills daily.

Labeling indicates alli is appropriate for anybody who is overweight, or has a body mass index of 25 or higher.

Just how many people find alli's benefits worth the cost of the drug is the "million-dollar question," said Kelly Brownell, a food policy researcher at Yale University.

Diet drugs don't deliver the big results most people expect, and are only effective when used along with diet and exercise, Brownell said.

To prepare for alli's launch in mid-June, the company ran TV spots directing viewers to a Web site where they could learn more about the drug. A retail book was also made available.