The Effect of Reduced Drug Copayments on Adherence to Oral Diabetes Medications Among Childless Adults in Wisconsin Medicaid

J Manag Care Spec Pharm. 2019 Dec;25(12):1432-1441. doi: 10.18553/jmcp.2019.25.12.1432.


Background: Medication adherence is an indicator of the quality of drug use, which is associated with better health outcomes and reduced health care expenditures. Drug cost sharing can be a barrier to adherence, especially for low-income individuals with chronic conditions. Most of the existing studies in a Medicaid population have evaluated the effects of increasing drug copayments, but few studies have evaluated the effects of reducing drug copayments on medication adherence. Medicaid coverage for low-income childless adults in Wisconsin was expanded on April 1, 2014, which included reductions in drug copayments and monthly caps on out-of-pocket spending.

Objective: To evaluate changes in adherence to oral diabetes medications using proportion of days covered (PDC) among Medicaid childless adults with type 2 diabetes after the 2014 Medicaid drug copayment reduction.

Methods: A difference-in-differences design was used to compare the changes in medication adherence between childless adults (treatment group) and parents/caretakers (control group). Wisconsin Medicaid's administrative enrollment records, pharmacy claims, and medical claims data were analyzed. Medication adherence was evaluated for 4 commonly used oral diabetes drug classes (i.e., biguanides, sulfonylureas, dipeptidyl peptidase-IV inhibitors, and thiazolidinediones) by adapting the medication adherence quality measures endorsed by the Pharmacy Quality Alliance. The PDC for all diabetes drugs was calculated among patients who filled ≥ 2 prescriptions for any of the 4 drug classes. PDC for each drug class was also measured among patients who had ≥ 2 drug fills for each drug class. The proportion of adherent patients was evaluated using a threshold of PDC ≥ 0.80.

Results: Average PDC for all diabetes drugs was 0.87 in the childless adults at baseline and significantly increased by 0.02 (P = 0.025) relative to the parents/caretakers after the copayment reduction. The baseline proportion of adherent patients (PDC ≥ 0.80) among the childless adults was 76% and significantly increased by 6.2 percentage points (P = 0.003) relative to the control group. The odds of adherence to oral antidiabetic drugs increased by 47%, resulting in the proportion of adherent patients in the childless adults group reaching almost 80% after the coverage expansion. In the per class analyses, a significant effect was found for biguanides; the proportion of adherent patients increased by 5.5 percentage points in childless adults compared with the control group (P = 0.022).

Conclusions: This program evaluation found that a reduction of drug copayments in Wisconsin Medicaid improved the quality of medication use by increasing adherence to oral antidiabetic drugs among childless adults.

Disclosures: This study was conducted as part of a larger study funded by the Wisconsin Department of Health Services. The authors are solely responsible for the content of this study. The authors report an evaluation contract with the Wisconsin Department of Health Services, unrelated to this study. An earlier version of this paper was presented at the AcademyHealth Annual Research Meeting; June 23-24, 2018; Seattle, WA.

MeSH terms

  • Adult
  • Diabetes Mellitus, Type 2 / drug therapy*
  • Diabetes Mellitus, Type 2 / economics*
  • Female
  • Humans
  • Hypoglycemic Agents / economics*
  • Hypoglycemic Agents / therapeutic use*
  • Male
  • Medicaid / statistics & numerical data
  • Medication Adherence / statistics & numerical data*
  • Medication Therapy Management / economics
  • Medication Therapy Management / statistics & numerical data
  • Middle Aged
  • Pharmaceutical Services
  • Pharmacy / statistics & numerical data
  • United States
  • Wisconsin
  • Young Adult


  • Hypoglycemic Agents