OBJECTIVE: To characterize the relation between HRT trial dissemination and age-specific HRT use, HRT discontinuation, and regional HRT variation before and after the trials’ publication.

DESIGN: Using the Medco Health database, we analyzed HRT prescription filling, discontinuation, and regional variation among women ≥55 years from May 1998 to May 2003.

MEASUREMENTS AND MAIN RESULTS: Approximately 340,000 women were eligible for Medco benefits each month. Within 3 months of the Women’s Health Initiative (WHI), HRT prescriptions declined from 12.5% to 9.4%, P≤.0001. When stratified by age, a statistically significant decline in HRT post-WHI occurred in all age groups, with the biggest decline among women ≥55 to 64 (18% to 11%, P≤.0001). Among HRT users, we found statistically significant increases in discontinuation in 2002 (67%) compared with 2001 (53%, P<.0001). Prior to the WHI there was substantial regional variation in HRT use, with the West South Central and mid-Atlantic having the highest and lowest proportions, respectively (19% vs 6%, P≤.0001). Despite a relative decline in HRT use of 25% to 42% across all regions, substantial geographic variation remained.

CONCLUSIONS: Hormone replacement therapy use decreased significantly immediately post-WHI, suggesting that trial results can have a rapid effect on practice. Marked regional variation in HRT use persisted after the WHI, suggesting that local practice patterns exert a strong effect on clinical behavior even after new evidence is available.