OBJECTIVES: Low-cost generic drug programs (LCGPs) offer an affordable way for individuals to obtain a wide variety of prescription drugs, but they may lead to exposure misclassification in administrative claims datasets. This study sought to assess the prevalence and determinants of LCGP use in a privately-insured adult population. METHODS: This study relied on data from the Medical Expenditure Panel Survey from 2005 – 2011. LCGP use was defined by two stipulations 1) The total cost of the drug was paid out of pocket and 2) The cost of the drug exactly matched the cost of an LCGP program. Demographics of LCGP users and non-users were compared. A multivariable logistic model was estimated to identify the determinant variables associated with LCGP use. RESULTS: Of the total study population (N = 25,288), 44.51% were users. The prevalence of use has increased over time, from 28.21% in 2006 to greater than 50% by 2011. On average, users were older (45.03 years vs. 41.36), had higher Charlson Comorbidity Indices (0.63 vs. 0.29), filled more prescriptions per person (33.62 vs. 14.53), and used more unique medications (7.33 vs. 3.80). The majority of users were female (59.79%), employed (81.56%), married (64.58%), white (76.06%), lived in urban areas (85.59%), had prescription drug coverage (96.92%) and fell into the two highest income categories (82.38%). Increasing age was a significant predictor of LCGP use as individuals 35-54 (OR 1.15, 95% CI 1.07 – 1.23) and 55-64 (OR 1.26, 95% CI 1.16 – 1.36) were more likely to be users compared to those aged 18-34. The odds of LCGP use increased nearly 20% (OR=1.19, 95% CI 1.18 – 1.20) for each additional unique medication used during an individual’s two-year study period. CONCLUSIONS: The prevalence of LCGP use in this privately insured adult population suggests a high potential for exposure misclassification in administrative claims datasets.