Pharmacy benefit management companies (PBMs) may indeed have the potential for cutting drug costs, but many worry that they might also have the potential for controlling physicians' prescribing habits. Over half the U.S. population receives drug benefits through PBMs, which often decide which drugs make it onto a formulary and which do not. Eighty percent of this market is controlled by just four PBMs: Medco (owned by Merck), PCS (Eli Lilly), Diversified (SmithKline Beecham) and Caremark (MedPartners).
Source: Pharmacy benefit management: savings or shackles? Health Care Business Digest. 1998;3(3):36–41.