Walmart and Sam's Club stores nationwide will soon stop selling e-cigarettes.

An internal memo obtained by CNBC said “regulatory complexity” and “uncertainty” were behind the decision.

The move comes as health concerns surrounding e-cigarettes mount. Some have linked them to a number of serious illnesses such as cancer and even deaths.

Michigan's governor, meanwhile, recently banned the sale of flavored e-cigarettes, claiming they appeal to underage smokers.

Walmart and Sam's Club stores will sell their existing inventory of e-cigarettes, the internal memo said. The shelf space they occupy will then be taken over by other products.

The American Vaping Association, a trade group, was critical of Walmart's decision.

“You know you are in the middle of a moral panic when big corporations like Walmart find it is easier to sell deadly combustible tobacco products than to sell harm reduction alternatives,” the president Gregory Conley told CNBC.

Earlier this year, Walmart raised the minimum age to buy tobacco products to 21, following the lead of several U.S. states.