Background and importance Oral anticancer therapy has advantages over intravenous chemotherapy, such as greater comfort for patients. However, the use of combination therapies or administration of concomitant medications to treat patients with comorbidities may increase the risk of drug interactions.

Aim and objectives To determine the prevalence, level of risk and type of potential drug–drug interactions in oncological outpatients treated with oral anticancer therapy.

Material and methods This was a retrospective observational study of 10 months’ duration (January 2019–October 2019). All patients who collected their oral anticancer drugs in the pharmacy service of a third level university hospital during the study period were included. Sociodemographic variables and active prescriptions in the last dispensing period were collected in the Abucasis programme. For the interaction analysis, the Lexicomp database was used, and interactions were classified as C (monitor therapy), D (consider therapy modification) or X (avoid combination).

Results In our study, 240 patients were included (53% women, mean age 63 years); 92.9% of patients were receiving treatment with one or more concomitant drugs in addition to cancer treatment. In 68% of these patients at least one potential drug–drug interaction was detected. Of the 657 interactions detected, in 128 (19.3%) a chemotherapeutic agent was involved: 63.3% classified as level C, 22.6% as level D and 14.1% as level X. In 72.7% of cases it was a pharmacokinetic interaction, which mainly affected absorption by modification of gastric pH or cytochrome P 450 enzymes, and in 27.3% there was a pharmacodynamic interaction, mainly additive effects of toxicity (such as an increased risk of myelosuppression or QTc prolongation). Corticosteroids, proton pump inhibitors, allopurinol, antiplatelets and oral anticoagulants were the drugs involved in the interactions classified as level X.

Conclusion and relevance The prevalence of potential drug–drug interactions in our patients was high, highlighting a high proportion of risk of level X interactions. Pharmacological interactions involved commonly used drugs in patients, which may compromise the efficacy of anticancer therapy and expose the patient to higher toxicity. After the study, the level X interactions were reported to the responsible physician.

References and/or acknowledgements No conflict of interest.