Roshini Raj, MD, is Health’s medical editor and co-author of What the Yuck?! The Freaky & Fabulous Truth About Your Body. Board-certified in gastroenterology and internal medicine, Dr. Raj is an assistant professor of medicine at New York University Medical Center and a contributor on the Today show. In our new book, Dr. Raj fields personal and provocative questions—about your body, sex, even celeb health fads.

Q: Is it safe to pack on and then lose weight rapidly like the stars do?

A: Julia Roberts told Entertainment Weekly that she gained 10 pounds while filming the scenes in Italy in the new chick flick Eat, Pray, Love. “If you look at any of the scenes of eating, by the end of the scene, I’m done eating. Like in the scene with the pizza, by the time the scene is over, I’ve eaten the entire piece. When we were in Naples, we started shooting at 8 in the morning, and I think by 8:45 I’d eaten 8 or 10 pieces of pizza. Pizza was what I ate all day that day,” she said.

Unlike Julia, who probably lost those pounds with a few extra cardio sessions, some stars are adding and dropping much larger amounts and much more quickly. Take rapper 50 Cent, who dropped over 50 pounds for his role as a cancer patient in Things Fall Apart. Or Russell Crowe, whose scale goes up and down for films such as Gladiator and Body of Lies. They have been fodder for many celebrity-magazine headlines.

Here’s the bottom line: It’s risky. Rapid weight gain can put a stress on your heart and joints. Rapid weight loss can damage your liver and cause gallstones, hair loss (due to low protein), and loss of muscle mass; in starvation mode (which happens when you restrict calories too severely), your body hangs on to fat and breaks down muscle. It can also lead to loose, hanging skin, which—while not dangerous—is unattractive and hard to get rid of without surgery.

Of course, celebs are under a lot of pressure to shed their movie-part pounds in time for the next awards show, but no one should be speed dieting. One to two pounds per week is the fastest anyone should be losing weight. Even celebrities.