Double-digit increases in pharmacy expenditures have induced health plan administrators to pursue alternative methods to control benefits. Shifting the burden to consumers in the form of higher prescription co-payments is one popular strategy. However, the impact of increased cost sharing is not always clear and may vary among therapeutic classes. OBJECTIVE: The objective of this study was to assess the impact of a $10 co-payment increase in a private, for-profit managed care organization. METHODS: This analysis examined changes in drug utilization one year prior to the co-payment increase with those one year after the increase for two therapeutic classes: non-sedating antihistamines and nasal steroids. Both descriptive statistics and paired t-tests were used to assess the impact of prescription utilization after the copay change. Also, relative price effects were measured as arc elastcities. RESULTS: The total number of prescriptions dispensed during the two years was 6,952 for non-sedating antihistamines and 1,806 for nasal steroids. After increasing the prescription co-payment, 470 more non-sedating antihistamine prescriptions were dispensed, although fewer refills were reported. In contrast, a total of 108 fewer nasal steroid prescriptions were dispensed after increasing the prescription co-payment. Similar to the antihistamines, fewer nasal steroid prescriptions were refilled in 1999. There was a statistically significant average decrease of 0.44 non-sedating antihistamine prescriptions noted in 1999, compared to 1998 (p<0.0001). In contrast, there was no change in the average number of nasal steroids dispensed in 1999 compared to 1998. Non-sedating antihistamine refills were less elastic with an unadjusted arc elasticity of -0.26 while nasal steroids were more responsive to the co-payment change with an unadjusted arc elasticity of -0.81. CONCLUSION: Based upon our results, a $10 dollar increase in patient cost sharing does not affect utilization equally across therapeutic drug classes.