In the United States, a pharmacy benefit manager (PBM) is a third-party administrator (TPA) of prescription drug programs for commercial health plans, self-insured employer plans, Medicare Part D plans, the Federal Employees Health Benefits Program (FEHBP), and state government employee plans. As of 2018 they have become industrial behemoths in the US health sector.According to the American Pharmacists Association, "PBMs are primarily responsible for developing and maintaining the formulary, contracting with pharmacies, negotiating discounts and rebates with drug manufacturers, and processing and paying prescription drug claims. For the most part, they work with self-insured companies and government programs striving to maintain or reduce the pharmacy expenditures of the plan while concurrently trying to improve health care outcomes."As of 2016, PBMs managed pharmacy benefits for 266 million Americans. PBMs operate inside of integrated healthcare systems (e.g., Kaiser or VA), as part of retail pharmacies (e.g., CVS Pharmacy or Rite-Aid), and as part of insurance companies (e.g., UnitedHealth Group). In 2016 there were fewer than 30 major PBM companies in this category in the US, and three major PBMs (Express Scripts, CVS Health, and OptumRx of UnitedHealth Group) comprise 78% of the market and cover 180 million enrollees.