"Zetacap" is being touted as a "gastric bypass pill," a far cheaper alternative to expensive gastric bypass surgery that shrinks your stomach from the size of a football to the size of an egg.
Last year, at least 140,000 gastric bypass operations were performed, with many patients experiencing dramatic weight loss. "American Idol" judge Randy Jackson, weatherman Al Roker and Sharon Osbourne all dropped more than 100 pounds thanks to gastric bypass -- and Carnie Wilson lost a whopping 188 pounds.
The company that makes Zetacap -- which costs $75 for a 30-day supply -- claims it contains a super-thick, non-digestible fiber with a secret ingredient that when combined with water forms a filling gel, like a balloon in the stomach, and within minutes the user feels like they've just eaten a big meal.
"[If] it works … I would have real concerns about that," said ABC News medical contributor Dr. David Katz. "What happens to the blob? How do you pass it out of there? There's a danger there. Intestinal obstruction is dangerous. You have to know if this forms a mass, that your system can break it down.
"More likely, it doesn't work at all," Katz added.
The company claims Zetacap will transform a person's body in 90 days. But diet experts say buyers should beware.
"I don't know that anybody could ever be transformed in any way, and that's also after gastric surgery," said Cathy Nonas, a registered dietician who is a spokeswoman for the American Dietetic Association. "You still have to do the hard work, eating smaller portions, eating a healthier diet."
The company that makes Zetacap says 200,000 users have lost a total of more than a million pounds by taking two pills with an eight-ounce glass of water before meals. The company's Web site claims the plan works for 90 percent of its users -- but the company did not return ABC News' calls.
"You can find testimonials for anything," Katz said. "I have no idea how many people who have taken this have benefited or how many were harmed. That concerns me. There's no information about adverse effects."
The pill doesn't require a prescription from a doctor. Because it contains all-natural ingredients, it hasn't been approved by the FDA -- nor does the FDA support any of the company's claims.
Most of Zetacap's ingredients have nothing to do with weight loss, Katz said. For example, the herbal blend contains St. John's wort, which is used to treat depression, Echinacea, which people use to fight colds, and valerian, which is used as a sedative.
"If this were to work at all, it would be because the fiber in here fills space in your stomach," Katz said. "We have evidence when you do that with food, it is beneficial."
Foods with high-fiber content -- fruits, vegetables and whole grains -- tend to be high in volume and low in calories, which helps control weight and appetite.
"I know that's healthy," Katz said, "but I have grave concerns about this [pill]."
ABC News' Andrea Canning reported this story for "Good Morning America Weekend Edition."