Drug persistency of two cholinesterase inhibitors : Rivastigmine versus donepezil in elderly patients with Alzheimer's diseaseObjectives: To compare persistency rates and persistency days in patients with Alzheimer's disease (AD) who initiated therapy with either rivastigmine or donepezil, and to identify factors influencing persistency in a real-world setting. Design and methods: This study used data collected by MarketScan® from 1 January 1999 to 31 December 2002. Patients were included if they were newly diagnosed with AD and filled at least one prescription for rivastigmine or donepezil between 1 July 2000 and 30 June 2001, were ?65 years of age on the index prescription date, and had continuous health and prescription insurance during the entire study period. Patients were excluded if they filled a prescription for any cholinesterase inhibitor during the 18 months prior to initiation of the study drugs. Patients who refilled their initial cholinesterase inhibitor prescription within a permissible gap of 60 days after depleting the drug supply from the prior prescription were considered to be persistent. Sensitivity analysis was performed to test the robustness of the persistency definition. The Kaplan-Meier method was used to determine persistency rates across time and Cox proportional hazards models were used to estimate relative risks of discontinuation or switch with adjustment for other covariates, and to identify factors significantly influencing persistency of the study drugs. Results: Of the newly treated AD patients, the proportion of rivastigmine and donepezil patients who continued their medication was the same (47%; p = 0.5). On average, rivastigmine users continuously used their medication for 234 days (median 312 days) while those taking donepezil used their medication for 235 days (median 315 days) [p = 0.91].