Potential drug–drug interactions in hospitalized patients undergoing systemic chemotherapy: a prospective cohort study

Background Adverse drug–drug interactions (DDI) are a major cause of morbidity and mortality in susceptible populations. Cancer patients are a population at high risk for DDI especially because they commonly receive several drugs concomitantly. The knowledge about the most common interactions between drugs used in oncology inpatients is essential to reduce drug-related problems and increase the safety and efficacy of the therapy. Objective To assess the frequency of potential DDI throughout the hospital stay of cancer patients undergoing systemic chemotherapy, describe their epidemiology, and identify risk factors for major DDI. Setting An oncology–hematology inpatient unit of a public hospital in southern Brazil. Method Drug prescriptions were prospectively reviewed throughout the hospital stay of patients admitted for systemic chemotherapy. Descriptive statistics and Poisson regression were used for data analysis. Main outcome measure Potential DDI and their characteristics. Results The cohort consisted of 113 patients, who used a mean of 8.9 ± 2.7 drugs/day. All patients had at least one potential DDI (median, 7.0/patient; 25th–75th percentile, 3.5–12.0), and 46 % of the patients had at least one DDI classified as major, i.e. that it may result in death, hospitalization, permanent injury, or therapeutic failure. Only 13.7 % of all interactions involved antineoplastic agents, identified in 62.8 % of patients. Most interactions were of moderate severity, 6.4 % were major, and 8.5 % had a recommendation for therapy modification. Multivariate analysis revealed mean number of drugs prescribed [relative risk (RR) for each additional drug: 1.12; 95 % confidence interval (CI) 1.07–1.17; P < 0.01] and age ≥60 years (RR 1.48; 95 % CI 1.03–2.14; P < 0.01) as independent risk factors for major DDI. Conclusion Potential DDI were highly frequent in this cohort. Older age and number of drugs prescribed were more likely to lead to major interactions. Prospective surveillance is required to detect adverse DDI, aiming primarily at reducing the risk of toxicity or treatment failure.