Background In the pharmacy there is a unit where certain drugs are dispensed to outpatients. Pharmacists provide pharmaceutical care to all patients starting new treatment.

Purpose 1) To identify and classify drug interactions (DIs) in all patients starting any treatment in the outpatient unit of our hospital pharmacy. 2) To carry out pharmaceutical interventions. 3) To identify patients who suffer more frequently from DIs.

Materials and methods Prospective intervention study (January–May 2011) that included patients who started treatment at the outpatient unit of the hospital pharmacy. Data were obtained from the medical prescription and an interview with the patient. To detect and classify DIs the authors used the software ‘Lexi-Interact-Online’ and the book ‘Stockley, Drug Interactions. Third Edition (2009). Data were analysed with SPSS-15.0.

Results Data collection comprised results from 104 patients (39 women, 65 men). Median age 53. SD 20 years (18–92). 187 DIs were detected (incidence 50.96%). 10.16% of the interactions occurred between drugs dispensed in hospital and concomitant home medicines (CHMs). 89.83% of the interactions detected were CHM-CHM. The risk of DI was rated as ‘major’ (14.43%), ‘moderate’ (79.14%) and ‘minor’ (6.43%). The reliability of the DI was ‘excellent’ (6.95%), ‘good’ (32.08%), ‘reasonable’ (57.75%) and ‘poor’ (3.22%). The mechanisms by which the DI developed were ‘pharmacokinetic’ (50.26%), ‘pharmacodynamic’ (35.82%), ‘unknown’ (12.3%), ‘other’ (1.09%) and ‘mixed pharmacokinetic/pharmacodynamic’ (0.53%). 38.46% of patients were polymedicated (≥6 drug). In these patients DI incidence was 87.70% versus a single drug in which DI was 12.3%. Pharmaceutical interventions were: monitoring DI from the outpatient unit in the pharmacy (10.6%), informing doctors (45.45%), advising on administration (31.81%), informing/educating patients (11.36%). Causes of non-intervention: habitual association/DI clinically irrelevant (51.93%), DI beneficial (25.12%), literature reports that there is no action required (12.59%), others (10.38%).

Conclusions 1) The appearance of DIs in patients starting treatment in the outpatient units is common. CHMs should also be reviewed to detect DIs. 2) Every DI must be assessed individually and appropriate pharmaceutical interventions made. Not all DIs are harmful or clinically relevant. 3) Polymedicated patients are a group of special interest because most of the interactions occur in them.