First popularized in the 1950s, the HCG diet is having another moment. But what is it, exactly?

Human chorionic gonadotropin (HCG) is a hormone produced by your body during pregnancy (it's what a home pregnancy test detects in your urine). As a drug, it's normally used in fertility treatment. The theory behind the HCG diet is that combining injections or supplements of the hormone with a very low-calorie diet (500 to 800 calories per day) will trigger the body to burn stored fat for fuel.

But HCG hasn't been approved by the FDA for weight control, and experts Health consulted are highly skeptical. Sure, you may lose weight at first, but only because you're eating so little; placebo-controlled studies do not suggest that HCG causes any more weight loss than diet alone. "There are no good controlled data," says Lee Kaplan, MD, PhD, director of the Massachusetts General Hospital Weight Center in Boston. "If you have a drug that doesn't clearly work, why would you take any risk?"