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HYPERTENSION

Generic Substitution of Antihypertensive Drugs: Does It Affect Adherence?

PhD Candidate, Department of Pharmacoepidemiology & Pharmacotherapy, Faculty of Pharmaceutical Sciences, Utrecht University, Utrecht, Netherlands

Olaf H Klungel, PharmD PhD

Associate Professor of Pharmacotherapy, Department of Pharmacoepidemiology & Pharmacotherapy, Utrecht University

Eibert R Heerdink, PhD

Asssistant Professor of Pharmacoepidemiology, Department of Pharmacoepidemiology & Pharmacotherapy, Faculty of Pharmaceutical Sciences, Utrecht University

Anthonius de Boer, MD PhD

Professor of Pharmacotherapy, Department of Pharmacoepidemiology & Pharmacotherapy, Faculty of Pharmaceutical Sciences, Utrecht University

Reprints: Dr. Klungel, Department of Pharmacoepidemiology & Pharmacotherapy, Faculty of Pharmaceutical Sciences, Utrecht University, Sorbonnelaan 16, 3584 CA Utrecht, Netherlands, fax 31 30 253 9166, o.h.klungel{at}pharm.uu.nl

BACKGROUND: Generic substitution is an important opportunity to reduce the costs of pharmaceutical care. However, pharmacists and physicians often find that patients and brand-name manufacturers have doubt about the equivalence of the substituted drug. This may be reflected by decreased adherence to therapy.

OBJECTIVE: To assess the association between generic substitution and nonadherence to antihypertensive drugs.

METHODS: We conducted a matched cohort study between January 1, 1999, and December 31, 2002. Data were obtained from PHARMO, a record linkage system containing drug-dispensing records from community pharmacies and linked hospital discharge records of approximately 950 000 people in the Netherlands. Residents of 30 medium-sized cities who initiated antihypertensive drug therapy were potential subjects. Refill adherence with antihypertensive drugs after substitution was determined; those with refill adherence below 80% were considered nonadherent.

RESULTS: Four hundred sixty-three patients with a substitution in therapy and 565 controls, matched on age, gender, therapy start date, duration of use, and generic product code, were identified. Of the patients who switched from brand-name to generic formulations ("substituted"), 13.6% were nonadherent, and of the non-substituted patients (those who did not switch to generic), 18.7% were nonadherent (OR 0.68; 95% CI 0.48 to 0.96). The association was absent in males. None of the patients discontinued the medication. No differences in hospitalizations for cardiovascular disease in the 6 months after the substitution were observed.

CONCLUSIONS: Generic substitution of antihypertensive drugs does not lead to lower adherence or more discontinuation and cardiovascular disease–related hospitalizations compared with brand-name therapy. When a less-expensive antihypertensive generic equivalent becomes available, generic substitution should be considered to achieve economic benefits.

Key Words: adherence, antihypertensive drugs, generic substitution

Published Online, November 22, 2005. www.theannals.com, DOI 10.1345/aph.1G163