Background Pharmacists may play an important role in the prevention of potential drug interactions (PDIs).

Purpose To investigate PDIs among intravenous cytotoxic drugs and medicines for comorbid illnesses in breast cancer patients, according to the interaction mechanism, its clinical significance and the published literature.

Materials and Methods Treatments for breast cancer patients were analysed in a retrospective study over a month. Data were collected from pharmacy oncology software (Oncowin) and the Primary care Prescription Data-Base (OMI-AP). Interactions were checked with Lexi-Comp Online.

Results 73 women were treated with intravenous cytotoxic drugs in November 2011. Mean age was 57 ± 13 years. Only 40 women were recorded in the Primary Care prescription database, and 3 of them did not receive concomitant treatment during that month. There were 10 different chemotherapy schemes involving 7 antineoplastic drugs. Comorbid chronic diseases were treated with 89 different drugs; antihypertensives, NSAIDs, benzodiazepines and antimicrobials were the most widely used drugs. 7 cases of PDIs were found, comprising 5 different interactions: cyclophosphamide/paroxetine (2), paclitaxel/diltiazem (1), docetaxel/trazodone (1), paclitaxel/atorvastatin (2), paclitaxel/ketoconazole (1). These interactions were detected in 6 patients (15% of patients with OMI-AP data). In one patient 2 PDIs were observed: cyclophosphamide/paroxetine and docetaxel/trazodone. All the PDIs detected were pharmacokinetic interactions. None of the PDIs detected had clinical relevance according to the scientific literature.

Conclusions PDIs may occur among drugs for chronic diseases and chemotherapy in breast cancer patients. These data are consistent with previous reports in which PDIs were observed in 19% of cancer patients. Most relevant interactions described are paclitaxel with antiepileptics, docetaxel with ketoconazole or cyclophosphamide with benzodiazepines. No clinically relevant interactions were found in our patients. Patients with comorbidities on multiple drug therapy (in addition to the drugs used for cancer treatment) would most benefit from pharmaceutical care.

No conflict of interest.