The risk and cost of drug-drug interactions in an older population acutely admitted to hospital in Ireland.

Background: Adverse drug reactions (ADRs) are associated with greater healthcare costs; drug-drug interactions (DDIs) are a common cause of ADRs.

Aim: To estimate the risk and cost of ADR-related hospital admission associated with DDI-exposure versus no DDI-exposure in an older community-dwelling population.

Method: This is a secondary analysis of a cohort study among 798 older individuals admitted acutely to hospital in Ireland (2016-2017). Full medication-history (pre-admission) was used to measure severe DDIs. Hospital costs were derived from unit costs. Logistic regression and propensity score weighting was used to examine the association between DDI-exposure and ADR-related hospital admission. Quantile regression was used to examine median costs associated with DDI-exposure. Adjusted odds-ratios (aORs), adjusted median costs, and 95% confidence intervals (CIs) are reported.

Results: N = 782 (98%) individuals using ≥ 2 drugs were included. Mean age: 80.9(SD ± 7.5) years; 52.2% female; 45.1% with an ADR-related admission. Pre-admission, n = 316 (40.4%) patients had any severe DDI, n = 466 unexposed; n = 113 (14.5%) had a DDI which increases bleeding risk, n = 669 unexposed. The risk of ADR-related hospital admission associated with any severe DDI was aOR = 1.02 [95% CI: 0.82, 1.28]), and aOR = 1.83 [95% CI: 1.35, 2.44]) for DDIs which increase bleeding risk. The median cost of ADR-related hospital admission associated with any severe DDI versus none, was €880 [- 1205, 3055]; and €3,520 [- 934, 7974] for a DDI which increases bleeding risk versus none.

Conclusion: DDIs which increase bleeding risk were associated with greatest ADR-related hospital admission risk and highest costs for the healthcare system. Further research examining these DDIs is needed.