Background and importance In recent years, new oral treatments for cancer and hepatitis C have been authorised. These treatments have in common that they present a high risk of clinically relevant drug–drug interactions (DDIs). Polypharmacy and the use of complementary and alternative medicines (CAM) are increasingly common.

Aim and objectives To determine the prevalence and type of DDIs between selected drugs dispensed in the outpatient pharmacy service and drugs or CAM that patients takes on a regular basis.

Material and methods A retrospective observational study was conducted. Inclusion criteria were patients treated with drugs with a high risk of clinically relevant DDIs for hepatitis C and oncohaematological malignancies between April 2018 and December 2019. Exclusion criteria were patients with other pathologies or treatments not considered ‘high risk’. Variables studied were: demographics (age, sex), reconciled drugs, type of DDI and pharmaceutical recommendations (PR). Data were obtained from the medical history, prescription chart and patient interview at the time of the first dispensing act. For the analysis of DDIs, the Lexi-comp interaction database and About Herbs, Botanicals and Other Products were used.

Results 130 patients (59% men), median age 62 years (range 25–87), were included. Diagnosis was oncohaematologic malignancy in 84% of patients and hepatitis C in 16%. Median number of drugs and CAM reconciled per patient was 6 (range 0–17). Drugs dispensed in the outpatient clinic were: 17% capecitabine, 14% temozolomide, 14% dabrafenib/trametinib, 11% glecaprevir/pibrentasvir, 8% imatinib and 32% others. DDIs were detected in 45% of patients: 29% type X (avoid combination), 29% type D (consider treatment modification) and 42% type C (monitor). The therapeutic groups that patients took on a regular basis involved the following: 25% antipyretic analgesics (metamizole), 22% proton pump inhibitors, 10% HMG-CoA reductase inhibitors, 8% oral antidiabetics and 34% others. DDIs with CAM were detected in 6% of patients. PRs were accepted, implemented in all cases and recorded in the patient‘s medical history: discontinuation of treatment (26%), switch to therapeutic equivalent (24%), analytical monitoring (27%) and clinical follow-up (23%). In all CAM cases with DDIs, it was recommended to stop the medicinal plant.

Conclusion and relevance A high prevalence of moderate/severe interactions was observed in patients treated with oral antineoplastic and antiviral agents. This highlights the importance of continued pharmaceutical care and patient interview.

Conflict of interest No conflict of interest