The FTC found that the three largest PBMs — CVS Caremark, Cigna Group’s Express Scripts and UnitedHealth Group’s Optum Rx — now manage nearly 80 percent of prescriptions filled in the United States.

PBMs leverage their management of formularies, or the list of drugs available on insurance plans, to negotiate rebate payments from drug makers that are supposed to reduce costs for patients and insurers. However, when doctors prescribe costly drugs that do not appear on an insurer’s formulary, patients can be forced to pay the full price out of pocket.

Earlier this year, New Jersey resident Ann Lewandowski sued her former employer, Johnson & Johnson, after the company’s insurance plan left her facing a $10,000 bill for a three-month supply of a name brand drug for treating multiple sclerosis. A generic version of the drug can be purchased without insurance at a cost between $28 and $77 at major pharmacies, according to the lawsuit, but these options were not available due to the PBM policy.