OBJECTIVES: Generic Drug Discount Programs (GDDPs), introduced in 2006 and offered by the majority of retail pharmacies nationwide, offer many commonly used medications at low out-of-pocket prices. The objective of this study was to estimate the proportion of prescription claims filled using a GDDP for four commonly used medications.  METHODS: The Medical Expenditure Panel Survey annual prescribed medicines event file, a nationally representative sample that contains detailed drug information including payments made by private insurance, Medicaid/Medicare, out-of-pocket, and other sources, was used for this study. Annual prescription claim records, including new fills and refills of any quantity dispensed, were estimated from 2006-2012 for all branded and generic monotherapy formulations of lisinopril, hydrochlorothiazide, metformin, and levothyroxine. Claims were considered GDDP-filled if the following criteria were met: the only recorded price paid was patient out-of-pocket, and the quantity dispensed and out-of-pocket price-paid matched published GDDP pharmacy lists, including Walmart, Walgreens, CVS, RiteAid, and Kroger. RESULTS: In 2006, the percentage of GDDP-filled prescriptions was low: 8.1% (N=4,472,797) of hydrochlorothiazide, 4.0% (N=3,880,992) of levothyroxine, 0.01% (N=4,008) of metformin, and 0.0% of lisinopril. This increased in 2008, to 2.4% (N=1,676,928) for metformin, 8.0% (N=7,107,840) for lisinopril, 16.8% (N=9,024,532) for hydrochlorothiazide, and 18.9% (N=13,035,972) for levothyroxine. In 2009, all medications had a GDDP-filled rate around 20% or greater. The highest percentages were seen in 2010: 22.5% (N=12,033,009) of hydrochlorothiazide, 25.1% (N=27,033,234) of levothyroxine, 25.5% (N=17,593,051) of metformin, and 29.6% (N=30,115,329) of lisinopril. By 2012 this decreased, and 19.6% (N=9,715,614) of hydrochlorothiazide, 20.3% (N=20,813,050) of lisinopril, 22.1% (N=21,425,226) of levothyroxine, and 23.2% (N=17,842,540) of metformin prescriptions were GDDP-filled. CONCLUSIONS: By 2012, approximately 1 in 5 prescriptions for lisinopril, hydrochlorothiazide, metformin, and levothyroxine were filled using a GDDP. As they are cash-only, these prescriptions may not be processed via a pharmacy benefit manager, and therefore may be missing from insurance claims data.