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.

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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."