Juul Labs Inc. plans to cut roughly 500 jobs by the end of the year, according to people familiar with the matter, reversing the embattled e-cigarette maker’s rapid staff growth as the company braces for a proposed ban on flavors that make up more than 80% of its U.S. sales.

The number of positions to be eliminated could range from 10% to 15% of the workforce but isn’t final, the people said.

The...