What is the difference between an MCO and a PBM?

An MCO is a managed care organization. A PBM is a Pharmacy Benefit Management Company. MCO's are basically insurance plans and include HMO's, PPO's, and POS plans. PBM's are usually a third party administrator (TPA) of a prescription drug program, but can sometimes be a service within an integrated healthcare system, such as a hospital or the VA. They are mostly responsible for processing and paying prescription drug claims, but they also contract with pharmacies and negotiate discounts and rebates with drug manufacturers. In the United States, there are currently fewer than 100 PBM's.