Thousands of people are latching onto a diet that promises rapid weight loss-up to 30 pounds a month-and, judging by its recent surge in popularity, actually delivers. But the so-called HCG diet is either a weight-loss miracle or a dangerous fraud, depending on who's talking. The plan combines drops or injections of HCG , a pregnancy hormone, with just 500 calories a day. While some believers are so convinced of its power they'll willingly stick themselves with a syringe, the government and mainstream medical community say it's a scam that carries too many health risks and doesn't lead to long-term weight loss.

HCG is approved by the U.S. Food and Drug Administration to treat infertility in both men and women. But its weight-loss roots trace back to the 1950s, when British endocrinologist A.T.W. Simeons realized that giving obese patients small, regular doses of the hormone helped them lose stubborn clumps of fat. It only worked, however, when coupled with a near-starvation diet. Simeons began touting HCG as a potent appetite suppressant that would make anything more than 500 daily calories unbearable. And he claimed the hormone could blast fat in key trouble spots like the upper arms, stomach, thighs, and buttocks, while preserving muscle. Save for a few tweaks, the modern-day incarnation is largely as Simeons presented it: Dieters supplement an extremely low-calorie meal plan with daily injections prescribed off-label by medical professionals, or take diluted, homeopathic HCG- typically in drop form-sold online, in drugstores, and at nutritional supplement stores.

Exactly why the HCG diet is experiencing a revival now is unclear, but the hype has sparked a response from the FDA. In January, the agency warned that homeopathic HCG is fraudulent and illegal when sold for weight-loss purposes. Though the FDA said such products aren't necessarily dangerous, their sale is deceptive, since there's no good evidence they're effective for weight loss. What's more, all HCG products, including injections prescribed by a doctor, must carry a warning stating there's no proof they accelerate weight loss, redistribute fat, or numb the hunger and discomfort typical of a low-calorie diet.

Though HCG dieters have some leeway in how they spend their 500 daily calories, they're urged to choose organic meats, vegetables, and fish. Dairy, carbs, alcohol, and sugar are all off limits. A day's meals might consist of coffee and an orange for breakfast; a little tilapia and raw asparagus for lunch; a piece of fruit in the afternoon; and crab, spinach, Melba toast, and tea for dinner. If dieters slip up, they're encouraged to compensate by drinking only water and eating nothing but six apples for 24 hours. That's thought to help squeeze out water weight, a psychological boost to help them get back on track.