Trends and Determinants of Polypharmacy and Potential Drug-Drug Interactions at Discharge From Hospital Between 2009-2015.

Background: Polypharmacy (PP) and excessive polypharmacy (EPP) are increasingly common and associated with risk of drug-drug interactions (DDIs). We aimed to measure the trends and determinants of PP and DDIs among patients discharged from the Department of Internal Medicine of the Lausanne University Hospital.

Methods: The retrospective study included 17,742 adult patients discharged between 2009 and 2015. Polypharmacy and EPP were defined as the concomitant prescription of five or more and ten or more drugs, respectively. Drug-drug interactions were defined as any combination of a drug metabolized by a cytochrome P450 or P-glycoprotein, and a drug considered as strong inductor or inhibitor of the corresponding enzyme was defined as a potential interaction.

Results: Three most commonly classes of drugs prescribed were "alimentary tract and metabolism (including insulins)," "nervous system," and "blood and blood forming organs." Polypharmacy decreased from 45% in 2009 to 41% in 2015, whereas EPP increased from 40% to 46%. In 2015, 13% of patients received 15 or more drugs. Age, coming from other health care settings, higher Charlson Index, number of comorbidities, and quartiles of length of stay were significantly and independently associated with PP and EPP. The risk of having at least one DDI decreased from 67.0% (95% confidence interval = 64.8-69.0) in 2009 to 59.3% (57.6-62.0) in 2015 (P < 0.001). Multivariate analysis showed number of drugs (odds ratio and 95% confidence interval = 3.68 [3.3-4.1], 9.39 [8.3-10.6], and 20.5 [17.3-28.4] for [5-9], [10-14], and 15+ drugs, respectively), gastrointestinal disease (3.13 [2.73-3.58]), and cancer (1.37 [1.18-1.58]) to be positively associated, and lung (0.82 [0.74-0.90]) and endocrinological (0.62 [0.52-0.74]) diseases to be negatively associated with risk of DDI.

Conclusions: The pattern of drug prescription has changed and most prescribed groups increased during the study period. Excessive polypharmacy is increasing among hospital patients. The decrease in the overall risk of DDI could be due to an improved management of multidrug therapy.