Prevalence of potentially serious drug–drug interactions among South African elderly private health sector patients using the Mimica Matanović/Vlahović-Palčevski protocol

Objectives To determine the prevalence of potentially serious drug–drug interactions (DDIs) and their relationship with gender and age, among elderly in South Africa. Methods A cross-sectional study was conducted using pharmaceutical claims data for 2013, for a total of 103 420 medical scheme beneficiaries’ ≥65 years. All medications dispensed within one calendar month where the days’ supply of medication dispensed overlapped, were grouped as one prescription. DDIs per prescription were then identified using the Mimica Matanović/Vlahović-Palčevski DDI protocol. Results were interpreted using effect sizes, that is Cramér’s V, Cohen’s d and Cohen’s ƒ2. Key findings A total of 331 659 DDIs were identified on 235 870 (25.8%, N = 912 713) prescriptions (mean 0.36 (SD 0.7) (95% CI, 0.36 to 0.37)). Women encountered 63.5% of all DDIs. Effect sizes for the association between DDIs and age group (Cramér’s V = 0.06), and gender (Cramér’s V = 0.05) was negligible. There was no difference between men and women regarding the mean number of DDIs identified per prescription (Cohen’s d = 0.10). The number of medicine per prescription (ƒ2 = 0.51) was the biggest predictor of the DDIs. The most frequent interacting drug combinations were between central nervous system medicines (30.6%). Conclusion Our study is the first to report the prevalence of potentially serious DDIs among an elderly population in the South African private health sector utilising the Mimica Matanović/Vlahović-Palčevski DDI protocol. Overall, we identified DDIs in approximately 26% of the prescriptions in our study. Age and gender were not found to be predictors of potentially serious DDIs.