• William H Shrank, Margaret Stedman, Susan L Ettner, Dee DeLapp, June Dirstine, M Alan Brookhart, Michael A Fischer, Jerry Avorn, and Steven M Asch.
• Division of Pharmacoepidemiology and Pharmacoeconomics, Brigham and Women's Hospital, Harvard Medical School, 1620 Tremont St, Suite 3030, Boston, MA 02120, USA. wshrank@partners.org
• J Gen Intern Med. 2007 Sep 1; 22 (9): 1298-304.

BackgroundIncreased use of generic medications conserves insurer and patient financial resources and may increase patient adherence.ObjectiveThe objective of the study is to evaluate whether physician, patient, pharmacy benefit design, or pharmacy characteristics influence the likelihood that patients will use generic drugsDesign, Setting, And ParticipantsObservational analysis of 2001-2003 pharmacy claims from a large health plan in the Western United States. We evaluated claims for 5,399 patients who filled a new prescription in at least 1 of 5 classes of chronic medications with generic alternatives. We identified patients initiated on generic drugs and those started on branded medications who switched to generic drugs in the subsequent year. We used generalized estimating equations to perform separate analyses assessing the relationship between independent variables and the probability that patients were initiated on or switched to generic drugs.ResultsOf the 5,399 new prescriptions filled, 1,262 (23.4%) were generics. Of those initiated on branded medications, 606 (14.9%) switched to a generic drug in the same class in the subsequent year. After regression adjustment, patients residing in high-income zip codes were more likely to initiate treatment with a generic than patients in low-income regions (RR = 1.29; 95% C.I. 1.04-1.60); medical subspecialists (RR = 0.82; 0.69-0.95) and obstetrician/gynecologists (RR = 0.81; 0.69-0.98) were less likely than generalist physicians to initiate generics. Pharmacy benefit design and pharmacy type were not associated with initiation of generic medications. However, patients were over 2.5 times more likely to switch from branded to generic medications if they were enrolled in 3-tier pharmacy plans (95% C.I. 1.12-6.09), and patients who used mail-order pharmacies were 60% more likely to switch to a generic (95% C.I. 1.18-2.30) after initiating treatment with a branded drug.ConclusionsPhysician and patient factors have an important influence on generic drug initiation, with the patients who live in the poorest zip codes paradoxically receiving generic drugs least often. While tiered pharmacy benefit designs and mail-order pharmacies helped steer patients towards generic medications once the first prescription has been filled, they had little effect on initial prescriptions. Providing patients and physicians with information about generic alternatives may reduce costs and lead to more equitable care.

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