A longitudinal analysis of antihypertensive drug interactions in a medicaid population

Background: Drug interactions are a frequent cause of adverse drug events. We evaluated whether the frequency of previously reported antihypertensive drug-drug interactions could be reduced by pharmaceutical case management. Methods: Patients >30 years of age with hypertension who were enrolled in the Iowa Pharmaceutical Case Management (PCM) program were evaluated. All prescription claims for patients were obtained on their date of eligibility and again 9 months later. A drug interaction database was developed to examine potential drug interactions in each patient's regimen. Results: Antihypertensive drugs were taken by 1377 patients at baseline and at 9-month follow-up. Highly significant antihypertensive drug interactions were observed at baseline in 35% of patients (0.47 per patient), and interaction prevalence did not change over time. Decreases in the number of drug interactions tended to occur more commonly among patients of pharmacies that provided the highest intensity of service (11.5% in high-intensity pharmacies v 9% in low- or zero-intensity pharmacies, but this did not achieve statistical significance). Nearly 75% of patients had an interaction of any significance level, and the total number of interactions increased over time (P = .0067). Conclusions: This Medicaid population with hypertension had a very high prevalence of potential drug interactions. The prevalence of interactions did not change, but the mean number of all interactions actually increased over time. There was some suggestion that higher-intensity pharmacies might be more successful in minimizing the risk of clinically significant drug interactions when compared with lower-intensity pharmacies.