['Patients with cancer often receive chemotherapeutic agents concurrently with other medications to treat comorbidity. The practical effects of concomitant medications, especially polypharmacy, on adverse drug reactions related to irinotecan-based chemotherapy are not well documented.', 'Associations of adverse drug reactions related to irinotecan monotherapy or a combination of irinotecan, 5-fluorouracil, and L-leucovorin (FOLFIRI) with concomitant medicines used to treat comorbidity were retrospectively investigated in Japanese patients with cancer.', 'Of the 172 patients, 118 received concomitant medications. Twenty-one patients had grade 4 neutropenia and/or grade 3 or 4 diarrhea. Univariate and multivariate analyses revealed that concomitant medications were significantly associated with irinotecan-related severe neutropenia and/or diarrhea (P = 0.023 and 0.044). Multiple concomitant medications were significantly related to severe irinotecan-related toxicity in patients given monotherapy or FOLFIRI (P = 0.01). The incidence of severe irinotecan-related toxicities increased in parallel with the number of concomitant medications.'] Is concomitant polypharmacy associated with irinotecan-related adverse drug reactions in patients with cancer?