Drug Class, Type of Pharmacy Influence Medication Substitution

Pharmacy TimesAugust 2020
Volume 88
Issue 8

Although study results show that substitution rates for most drug classes exceeded 90%, some were as low as 64%.

The use of generic medications has been found to conserve financial resources and often increase patient adherence. However, the rates of substitution can vary based on several factors, including drug class, the number of prescriptions a patient has, and the type of pharmacy dispensing the medication.1

Investigators conducted a cross-sectional analysis of data and quantified generic substitution rates for 26 drug classes, according to findings published in Therapeutic Innovation & Regulatory Science.1

For 8 drug classes with low generic use, they used mixed effects logistic regression to estimate relationships between the determinants of interest and generic substitution rates.1 Notably, the generic substitution rates for most of the drug classes were more than 90%, meaning that many brand-name drugs are being substituted for equally efficacious and more affordable options. Some classes, however, had much lower substitution rates, including androgens (74%), estrogens (71%), hydantoin-type anticonvulsants (72%), and thyroid hormones (64%).1

Although determinants varied across the drug classes, the investigators found several notable patterns. For 5 of the 8 drug classes studied, patients using a mail order pharmacy had a lower rate of generic drug substitution than those getting their prescriptions from retail pharmacies.1 Similarly, men were more likely than women to receive generic substitutions, and patients taking more medications were also more likely to have generic drug substitutions.1

In addition to financial implications, prescribing generic drugs or giving patients a choice in their prescription can also have a vital impact on patients’ adherence to their medications and their overall health.

According to an analysis of 7532 prescriptions filled between October 2001 and October 2003, 18.7% were for generics, 58.1% were for preferred drugs, and 23.2% were for medications not preferred by the patient.2 The investigators found that the proportion of days covered (PDC) was 12.6% higher for patients initiated on a generic medication compared with those initiated on nonpreferred medications. Similarly, the PDC was 8.8% greater for patients on preferred versus nonpreferred medications.2

Some states have implemented mandatory generic substitution laws requiring pharmacists to substitute a generic for a brandname drug if the prescriber did not specify which one should be dispensed. Additionally, some states require patient consent before substituting a generic alternative, giving those patients more influence over their medications.3

The investigators analyzing determinants of generic substitution noted that state substitution laws and patient consent laws did not appear to have much effect on generic substitutions.1

Nevertheless, they concluded that more policies are necessary to support the use of effective, lower-cost, and safe generic drugs whenever they are available in an effort to minimize financial costs and support patient health and medication management.1


  • Segal JB, Onasanya O, Daubresse M, et al. Determinants of generic drug substitution in the United States. Ther Innov Regul Sci. 2020;54(1):151-157. doi:10.1007/ s43441-019-00039-9
  • Shrank WH, Hoang T, Ettner SL, et al. The implications of choice: prescribing generic or preferred pharmaceuticals improves medication adherence for chronic conditions. Arch Intern Med. 2006;166(3):332-337. doi:10.1001/archinte.166.3.332
  • Shrank WH, Choudhry NK, Agnew-Blais J, et al. State generic substitution laws can lower drug outlays under Medicaid. Health Aff (Millwood). 2010;29(7):1383- 1390. doi:10.1377/hlthaff.2009.0424

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