The U.S. Consumer Product Safety Commission on Thursday recalled small but powerful magnetic balls used as building blocks and stress relievers, citing thousands of hospitalizations and at least seven deaths due to similar products.

The Relax magnetic balls are sold exclusively online at Walmart in a set of 216 multi-colored balls that each measure five millimeters. More than 4,200 units are being recalled.

They were sold there from February 2022 through April 2023 and cost between $14 and $15, according to the recall notice.

The safety commission said it “estimates 2,400 magnet ingestions were treated in hospital emergency departments from 2017 through 2021.”

The commission knows of “seven deaths involving the ingestion of hazardous magnets, including two outside of the United States,” according to the recall notice.

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The safety commission added, the “magnetic ball sets do not comply with the requirements of the mandatory federal magnet regulation because the sets contain one or more magnets that fit within CPSC’s small parts cylinder and the magnets are stronger than permitted.”

Swallowing a magnet is dangerous because they are so high-powered, they can attract to each other or to another metal objecte and become lodged in the digestive system, according to the safety commission.

“This can result in perforations, twisting and/or blockage of the intestines, infection, blood poisoning, and death,” the recall notice said.

The magnetic balls are manufactured in China and sold through a company called Joybuy.

No one with Joybuy could be immediately reached Thursday afternoon for comment.