Sometimes, you can’t stop your weight-loss secrets from leaking out.

Dieters have been flocking to drugstores to pick up Alli, the first over-the-counter weight-loss pill to be approved by the Food and Drug Administration, despite the scary warning: Stray too far from your low-fat diet and you just might poop your pants.

The drug’s maker, GlaxoSmithKline, has been up front about the pill’s side effects, suggesting that first timers wear dark pants or bring a change of clothes to work until they get used to the diet pill’s potentially yucky consequences.

Still, it seems there’s no shortage of people willing to risk public humiliation to shed a few pounds. At one Los Angeles-area Walgreens, pharmacist Susie Uyu’s seen customer after customer march directly through the store toward the prominent Alli display. “I think they’re excited that it’s an over-the-counter product,” Uyu says.

And even though pharmacist Miyuki Anderson, who works at a Bartell Drugs in Seattle, warns everyone who eyes the Alli display about the messy side effects, it doesn’t stop most of them from buying the diet pill. (Anderson does, however, arm them with this helpful tip: “I tell the patients, try when you have a day off.”)

“We know it’s selling very well — better than we expected,” says Brian Jones, a GlaxoSmithKline spokesman. Jones declined to share any specific numbers. “But we don’t know if it’s going to last — there was a lot of pent-up anticipation.”

Anyone can try it

That anticipation refers to the origin of Alli; it’s the newly approved over-the-counter form of the prescription weight-loss drug Xenical. Now that it’s available in many major drugstores and grocery chains, it’s not just for the obese with a doctor’s prescription in hand — anyone who wants to lose a few can try it.

“The pill offers the promise of convenience, that someone has done the job for you,” says Adam Drewnowski, director of the Center for Public Health Nutrition at the University of Washington in Seattle. “People who don't live well, who stuff themselves with bags of snacks, in desperation they reach out for a pill.”

The drugmaker states very clearly that it’s no miracle drug, and only promises to help people toward moderate weight loss. For example, if someone were to lose 10 pounds from dieting, they’d lose 15 by combining their diet with Alli.

The diet pill works by blocking 25 percent of fat from being digested. Alli users take one pill with every meal, and to avoid an “Alli oops,” they should eat less than 42 grams of fat a day, or about 15 grams per meal. But those fat grams can be sneaky. One grande Starbucks Caramel Frappuccino contains 15 grams of fat, and if an Alli user adds even a low-fat muffin to that meal, it could get icky.

“It’s so important to understand that you must adopt a low-fat, healthy lifestyle,” Jones says. “We call them treatment effects — that’s a signal for you that you’re not staying in the guidelines. What Alli will not do is make up for not living a healthy lifestyle.”

Cheaters share cautionary tales

But we don’t always like to bother with directions. Those who haven’t completely followed instructions offer cautionary tales on the drug company’s Web site.

“(I)’ve pooped my pants 3 times today, and sorry to get descriptive but it even leaked onto the couch at one point!” writes one user.

It can strike any time — even in the early hours of the morning. One user writes: “(Y)a know how when you start moving around in the morning ya pass a little gas. Well, I did and then went into the bathroom and to my horror I had an orange river of grease running down my leg.”

Fellow cheaters advise each other on the best clean-up methods, and some even suggest using panty liners or Depends. One frugal user noted, “I’m thinking that infant diapers might be a cheaper way to go, just use them as a large pad.”

The gross side effects might scare away the less-committed, but some experts appreciate Alli’s very real, very immediate consequences of cheating on your diet.

“It forces you to eat a lower-fat diet — if you don’t, you’re violently penalized for not doing so,” says David Sarwer, the director of clinical services at the Center for Weight Loss and Eating Disorders at the University of Pennsylvania School of Medicine. “When they eat a little too much fat, they’ll learn not to do it again.”

The drugmaker claims Alli is promoting healthier lifestyles by teaching users that weight loss involves eating healthy food and getting enough exercise — and Sarwer agrees.

“People who are struggling with their weight assume that thin people never think about what they eat,” Sarwer says. “I’ve always been impressed by patients who really rolled with the punches with some of these events. They say, ‘Well, I learned that I couldn’t do that. It taught me to eat differently.’ And that’s where I think Alli can be the greatest benefit.”

Side effects are avoidable

Some Alli enthusiasts have been conscientious enough to avoid any side effects. Carole McMahan, who’s trying to lose 10 pounds, started taking Alli the day the product hit drugstore shelves on June 15, and has been careful to follow the low-fat diet.

“No pun intended, but I’m very anal about it,” says McMahan, who’s 44 and lives in Greensboro, N.C. She appreciates the way Alli holds her accountable to her eating habits.

“I started very cautiously, and I’ve just grown more and more comfortable with it,” McMahan says. “I just follow the diet. I knew I couldn’t go out and order hot fudge cake.”

But some Alli fans, like 25-year-old Rachelle Beaini, are just asking for it. Beaini, who lives in Henderson, Nev., and wants to lose 20 pounds, has lost 6 pounds in two weeks without a single side effect. Inspired by her success, last week she dared to eat a meal of chicken nuggets — while wearing white pants. (Still no unpleasant consequences, she swears.)

Still, as some obesity experts point out, if you’ve made a change in your eating habits, why is a diet pill necessary? Drewnowski, the Seattle public-health researcher, says that hearing “Alli oops” stories frustrates him.

“I think it's utterly revolting, frankly,” Drewnowski says. “It controls your life — you focus on it all day. It’s like walking around with a colostomy bag.”

Instead of investing money in a diet pill, he encourages people to take the money they would’ve spent on Alli and put it toward buying healthier food. (A 20-day supply of Alli costs about $45 to $55.)

“See how much money you are spending on food daily,” Drewnowski says. “Factor in the extra amount for this drug and spend the entire amount on better food. You'll be happier and better nourished — and not obese.”

He adds, “I can't think that a healthy lifestyle requires carrying a spare pair of underpants.”