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.

“It’s reckless, irresponsible, and completely irrational,” says Pieter Cohen, an assistant professor of medicine at Harvard Medical School. “Can you lose weight on it? Of course, but that’s mainly because you’re hardly consuming any calories. And any benefit is not going to last.”

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.

Nonetheless, doctors are still doling out prescriptions for the daily injections, typically inserted into the thigh. At New Beginnings Weight Loss Clinic in Florida, for example, an in-house physician has prescribed injections to 3,000 clients since 2008, and clinical director Jo Lynn Hansen has recently observed a marked jump in interest. There, clients can opt for either a 23-day plan ($495) or a 40-day regimen ($595). After taking a six week break and eating normally—to prevent the body from becoming “hCG-immune”—many resume the process, completing multiple cycles. “We have people flying in from all over the country,” Hansen says. “It’s just a tiny little needle that pricks the skin. Anyone can do it.”

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.

“It wasn’t that hard to pull off, and I’d do it again in a heartbeat,” raved London-based fashion stylist Alison Edmond in February’s Marie Claire. “In the end, I lost a total of 25 pounds, ending up at a weight I hadn’t been in 10 years.” Despite success stories like hers, scientific evidence on the plan is shaky at best. In 1995, researchers analyzed 14 clinical trials on the hCG diet. Only two concluded hCG was any more effective than a placebo at helping people lose weight. And nearly 10 years earlier, a report in the Canadian Medical Association Journal stated hCG has “no value” as a means of managing obesity, and that the diet has been “thoroughly discredited and thus rejected by the majority of the medical community.”

Detractors say the hormone isn’t some miracle ingredient to weight loss—the restrictive diet is. “If you don’t eat, you lose weight,” Cohen says. “If hCG truly diminished hunger, it would be a wonderful drug. But if that were the case, why couldn’t you just modestly reduce your intake while using it? Why would you have to simultaneously starve yourself?” But believers insist that, thanks to hCG, they can stick to a low-calorie diet without hunger pangs, while losing unwanted fat. They’re adamant that hCG is essential to the diet’s success. “People are strongly convinced that this hormone will keep them on a 500-calorie diet. And the power of suggestion can be a very strong force,” says Cohen.

Of course, the regimen isn’t without risks. The hormone is known to cause headaches, blood clots, leg cramps, temporary hair thinning, constipation, and breast tenderness. The FDA has received at least one recent report of an HCG dieter developing a pulmonary embolism, a potentially fatal blood clot in the lung, says agency spokesperson Shelly Burgess. Yet, the hormone’s full risk profile is unknown. “HCG was studied briefly [for weight loss] and found to be ineffective, so we have no idea what its potential risks are,” Cohen says. “Do I have data that it causes heart attacks, stroke, or cancer? No, I don’t, because we just don’t know at this point.” While hCG may be safe on its own—the FDA says it’s safe as an infertility treatment—pairing it with an extremely low-calorie diet could have unexpected side effects.

Two years ago, Lori Hill, 40, of Salt Lake City, Utah, began a 28-day hCG diet cycle. She says she lost about 26 pounds, including thigh fat, largely without hunger. But she felt ill almost immediately, and by the last week of the diet, Hill—a fit and active soccer referee—couldn’t climb a flight of stairs without gasping for breath. The effort made her muscles burn and shake, too. After completing the cycle, Hill regained all the weight she had lost, plus an additional 15 pounds. “I starved myself and threw all my nutrients out of whack,” she says. “You’re tricking your body into letting you starve, without feeling any major hunger. What you’re doing to your body just isn’t worth it.”

There’s no question that 500 calories a day is tantamount to malnutrition—dieters should never dip below 1,200, say experts—and federal dietary guidelines recommend more than three times the amount of calories the diet prescribes for women ages 19 to 30. Moreover, extremely low-calorie diets can cause severe bone and muscle loss, electrolyte imbalances, gallstones, and even death. “I’ve heard a lot of people say the side effects of this diet are overwhelming,” says registered dietitian Keri Gans, a spokesperson for the American Dietetic Association. “And they could start as soon as one day in—you’ll start feeling irritated and tired.”

To Gans, the regimen is nothing more than a crash diet—and an expensive one at that. A more sensible route to weight loss, she says, is no more mysterious than choosing healthy foods, limiting portion sizes, and exercising. “This is another approach for people who believe there’s a silver bullet, but there is no such thing. All this diet does is show you how to restrict, and a person can only do that for so long without returning to old habits.”