Patient, Physician, Pharmacy, and Pharmacy Benefit Design
Factors Related to Generic Medication Use
William H. Shrank, MD, MSHS1, Margaret Stedman, MPH1, Susan L. Ettner, PhD4,
Dee DeLapp, RPh5,6, June Dirstine5,6, M. Alan Brookhart, PhD1, Michael A. Fischer, MD1,
Jerry Avorn, MD1, and Steven M. Asch, MD, MPH2,3,4
1Division of Pharmacoepidemiology and Pharmacoeconomics, Brigham and Women’s Hospital, Harvard Medical School, Boston, MA, USA;
2VA Greater Los Angeles Healthcare System, Los Angeles, CA, USA;3RAND Health, Los Angeles, CA, USA;4Department of Medicine, David
Geffen School of Medicine at UCLA, Los Angeles, CA, USA;5Anthem Blue Cross Blue Shield, North Haven, CT, USA;6Anthem Prescription
Management, North Haven, CT, USA.
BACKGROUND: Increased use of generic medications
conserves insurer and patient financial resources and
may increase patient adherence.
OBJECTIVE: The objective of the study is to evaluate
whether physician, patient, pharmacy benefit design, or
pharmacy characteristics influence the likelihood that
patients will use generic drugs
DESIGN, SETTING, AND PARTICIPANTS: Observation-
al 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 sepa-
rate analyses assessing the relationship between inde-
pendent variables and the probability that patients were
initiated on or switched to generic drugs.
RESULTS: Of 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 medica-
tions. 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
CONCLUSIONS: Physician 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 phar-
macy 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.
KEY WORDS: patient; physician; pharmacy; pharmacy benefit; design;
generic medications; medication costs.
© 2007 Society of General Internal Medicine 2007;22:1298–1304
In 2005, spending for prescription drugs in the United States
exceeded $250 billion.1The growth of prescription drug spend-
ing has outpaced spending in all other sectors of the health care
system in the last decade2and is predicted to continue to do
so.3Increased generic drug use is an important means of
controlling drug costs without compromising quality of care,4–6
and research suggests that overall drug spending could be
reduced by over 10% by using generics when available.7
In addition to reducing total expenditures for prescription
drugs, generic prescribing can have a substantial effect on
patients’ out-of-pocket costs and even adherence. In 2004,
approximately 68% of patients with prescription drug coverage
were enrolled in plans that required at least 3 tiers of
copayments.8Such plans generally require patients to pay
greater copayments for branded drugs than for generic drugs.
A recent study found that patients enrolled in tiered pharmacy
benefit plans are more adherent to chronic therapy when they
are initiated on generic medications than when they are
initiated on branded medications, suggesting that prescribing
generic drugs can improve medication-taking behavior while
Many factors may influence whether patients receive generic
medications. Physicians write prescriptions and substantially
An abstract of this paper was presented as a Hamolsky finalist at the
SGIM national meeting in Los Angeles, 2006.
Received December 28, 2006
Revised May 23, 2007
Accepted June 21, 2007
Published online July 24, 2007
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Shrank et al.: Factors Associated with Generic Drug Use