OBJECTIVE: To evaluate the usefulness of intervention in drug interactions of antiretroviral drugs with coadministered agents by a clinical pharmacist in outpatient HIV-treatment. METHODS: The study design included two intervention arms (A and B), which were both preceded by a control observation period. In arm A, a complete list of the currently used drugs, extracted from pharmacy records was provided to the treating physician. In arm B the same list was provided but with a notification when a drug interaction was present and an advice how to handle this. The infectious disease specialist obtained the information before the patient's visit to the outpatient clinic (time point 0). Three months prior (time point -3) and 3 months after (time point +3) the intervention, pharmacy records were also screened for drug interactions. The number of drug interactions (total and per patient) was determined at the three different time points (-3, 0, +3). In addition, drug interactions encountered at time points -3 and 0 were checked for their presence at time points 0 and +3, respectively, for both intervention arms. RESULTS: Arms A and B included 115 and 105 patients, respectively. Patient characteristics of both intervention arms were similar at time point 0. The number of interactions and the number of patients with interactions were similar in both intervention arms at time point 0. There were 42 and 40 potential drug interactions in 30 and 24 patients in arms A and B, respectively. The reduction in the number of interactions per patient over time and after intervention was small but significant, and was equal in both intervention arms. The advice of the clinical pharmacist had thus no additional value. CONCLUSION: Both interventions were effective in reducing the number of drug interactions per patient. The advice of a clinical pharmacist was, however, redundant in the studied setting.