The federal government is suing some pharmacy benefit managers over a system of drug rebates that regulators say has made the price of insulin soar for patients.

Insulin, used by millions of Americans, has taken center stage in the high-stakes battle over health care costs. Though prescribed for the past 100 years, only three companies supply insulin in the U.S. and much of that is higher-cost branded versions, despite the increasing availability of lower-price biologic options.