This excerpt taken from the CVS 10-K filed Feb 27, 2007.
Customers ~ Managed care and other third party plans accounted for 95% of our 2006 pharmacy revenues. Since our revenues relate to numerous payors, including employers and managed care organizations, the loss of any one payor should not have a material effect on our business. No single customer accounts for 10% or more of our total revenues.
We also fill prescriptions for many state Medicaid plans. Total revenues from all such plans were approximately 7% of total pharmacy revenues during 2006. Under the Medicare Modernization Act, Medicare eligible patients that were previously receiving Medicaid prescription coverage were automatically transitioned to Medicare coverage effective January 1, 2006. As a result we saw a decline as dual eligible participants transitioned from the higher margin Medicaid plans to lower margin Medicare Part D plans. However, we do not expect a further decrease of any significance in 2007.