Pharmacy benefit managers (PBMs) play an important role in the complex U.S. health care system. They sit between payors (e.g., insurance companies and employers) and the pharmacies, providing important services to both. Express Scripts Holding is the largest PBM in the country, after its recent merger with Medco Health Solutions. It handles the pharmacy benefit portion of health insurance plans, filling more than 1 billion prescriptions last year and capturing about 40% of the market. Despite its market-leading size, however, there is still room to grow.

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