If you watched Michael Phelps in the men's 4-by-100 relay at the Olympics last night, you probably noticed that the 31-year-old swimmer looked like he'd been pelted by balls from an angry pitching machine.

Those circular bruises on his shoulders came from cupping, a type of alternative medicine where cups are placed on the skin to create suction. The suction is said to increase blood flow to stimulate healing—similar to a massage—and many Olympians swear by it.

The British Cupping Society claims the therapy can be used to treat blood disorders, rheumatic diseases, fertility issues, skin problems, high blood pressure, migraines, anxiety, depression, and allergies.

But like most forms of alternative medicine, few scientific studies exist testing its effectiveness. So while Michael Phelps and other Olympians are onboard, the scientific jury is still out.

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