Medco is the top pharmacy benefits manager in the country, and getting bigger all the time. In 2010, the company filled 957 million prescriptions, up 6.5% from the previous year.The company lost a big client in March when the California Public Employee’s Retirement System (CalPERS) declined to renew its prescription drug contract for 2012. That loss may sting, but Medco should get a financial boost next year: The patents on several blockbuster drugs are expiring, with generic versions sure to follow. Generic drugs tend to be more profitable than brand names for Medco’s mail-order pharmacy service. —S.D.