There's no evidence cupping works. Why is it so popular?

Don't be surprised if your friends and neighbors are soon covered in purple spots.

"Cupping" is poised to become the latest fad.

Swimming champion Michael Phelps' use of cupping, a type of alternative medicine intended to ease muscle pain, has attracted nearly as much attention as his latest gold medal.

Cupping, which has been used in traditional Chinese medicine, involves using cups to create suction on the skin. Fans claim that pulling the skin away from the body improves their blood flow. What's not in dispute: The procedure leaves people covered in dark purple marks.

How cupping works and why Olympic athletes use it

Phelps, who won his 19th Olympic medal Sunday, said he relies on cupping to heal sore muscles. And he's not the only one. Track and field competitors in Rio are using it. So are male gymnasts at the Olympics. Celebrities Gwyneth Paltrow and Jennifer Aniston are big fans of the big dots.

But does it actually work?

There's little to no medical evidence that cupping has any benefit, said Robert Glatter, an emergency physician at Lenox Hill Hospital in New York and a former sideline physician for the New York Jets.

“There are studies on this, but they aren’t well done,” Glatter said.

In 2012, a review of 135 studies on cupping found it had some benefit for shingles, facial paralysis, acne and age-related wear and tear of the spinal disks of the neck. But authors of the review noted that the studies weren’t carefully done, so their results weren’t very valuable.

The review found cupping had no benefit for sore muscles.

That doesn't mean cupping is useless, Glatter said.

Cupping could work as a placebo, giving elite athletes a psychological boost, Glatter said. In other words, cupping works because people think it works. “When people feel better, they may perform better,” Glatter said. “But in terms of performance and power, (Phelps) already got that in the bag.”

While using a suction cup on sore muscles seems harmless, Glatter notes that people who heat the cups could potentially burn themselves. People could also develop infections.

“You’re causing tissue injury, and there could be bacteria on the skin,” Glatter said.

Cupping helps heal the USA men's Olympic gymnastics team

But cupping isn't going to turn the average person into an Olympian, Glatter said.

“It’s a hickey, to be honest,” Glatter said. “What you’re getting is a large, circular hickey.”