OBJECTIVES: Antidepressant medications are frequently used for the treatment of various psychiatric disorders, including depressive and anxiety disorders. Expenditure for antidepressants in 2001 was ranked as number one among all therapeutic categories in US. The objectives of this study were to examine antidepressant utilization trends and to understand market-share competition between brand-name and generic drugs. METHODS: Three major classes of antidepressants are selective serotonin reuptake inhibitors (SSRIs), tricyclic antidepressants (TCAs), and other antidepressants. Using the Center for Medicaid/Medicare Services (CMS) prescription drug database, we constructed quarterly per-prescription reimbursement figures for each brand-name and generic drug from 1991 to 2004. The market-share for each drug or class was calculated as the proportion of total number of antidepressant prescriptions. RESULTS: Total expenditure of antidepressants increased sharply from $18 million per quarter in 1991 to $350 million per quarter from 2001-2004. The proportion of total expenditure for brand-name drugs increased from 70% in 1991 to 94% in 2001. Brand-name market-share increased dramatically from 27% in 1991 to 50% in the second quarter of 1997, and then to 63% in the first quarter of 2001. In the third quarter of 2001, brand-name market-share dropped to 52%. SSRIs (Prozac®, Zoloft®, Luvox®, Paxil®) dominated the brand-name market. The market share for Prozac® decreased sharply from 23% in the third quarter of 2000 to 2% in the first quarter of 2004, due to its generic entry. Celexa's® market share decreased from 20% in the third quarter of 2002 to 15% in the first quarter of 2004, due to the market entry of Lexapro®. Generic antidepressant market-shares decreased over time, particularly those of TCAs. CONCLUSION: Large increases in antidepressant drug expenditures paralleled increases in brand-name market-share. Competition between brand-name and generic drugs was observed in the US Medicaid marketplace.