OBJECTIVES: 1) To characterize design of drug benefits of SSRI antidepressants in health plans offered by employers in the United States; and 2) To determine the effect of raising copayments on compliance rates of SSRI antidepressants. METHODS: Data comprised benefit information and claims from Medstat's MarketScan database for 2000-2003. Benefit information were compiled from approximately 135 different plans. Any patient who filled a prescription SSRI antidepressants in 2000 and was continuously enrolled through 2001 was identified. A difference in difference approach was used to examine the change in the days supplied and number of claims filled for an employer that raised their three tiered co-payments as compared to an employer that kept constant one tier copayment rates. RESULTS: Three tier copayment structures were increasingly common among employers. Most SSRIs fall in tier two although some of the newer SSRIs are commonly found in tier three. The average copayment for tier 1 increased from $5.40 to $7.40. The average copayment for tier 2 increased from $13.60 to $16.80. The average copayment for tier 3 increased from $25.40 to $31.20. When the study employer raised their co-payments by 50%, they experienced a 25% decline in the number of prescriptions per person filled (from 5.2 to 3.9 prescriptions) from 2000 to 2001, while the control employer demonstrated a 20% decline (from 6.0 to 4.8) in the number of prescriptions filled. Days supplied fell by 41.3 days or 24% in the employer that raised copayments and by 36.3 days or 17% in the control employer. CONCLUSION: Benefit structure and co-pays have trended towards 3-tier plans with increasing copayments. As such, increasing copayments may have a negative effect on compliance and possibly outcomes.