Antibiotics are undeniably beneficial. However, inappropriate or incorrect use puts patients at risk for avoidable adverse drug reactions, promotes emergence of resistance and potentially increases overall health-care costs. The objective of this study was to assess the impact of pharmaceutical consulting on the quality and costs of antibiotic use in surgical wards. From February 2007 to February 2008 a total of 638 patients were enrolled in the controlled intervention study. Within the control period (n = 317) the current pattern of anti-biotic use was monitored without intervening, in the intervention period (n = 321) the pharmacist gave advice with regard to optimised antibiotic therapy. In 216 patients 331 antibiotic-related problems were identified; 232 interventions resulted in a modification of therapy (acceptance 70 %). The most common interventions were those regarding the duration of therapy and the choice of agent. The intervention with the greatest acceptance (91 %) was dosing recommendations. The pharmaceutical intervention resulted in a shorter duration of therapy (9.9 vs. 11.2 days, p < 0.001) and an increased adherence to the surgical department's guidelines (64 % vs. 71 %, p = 0.03). Intravenous therapy was switched to oral therapy earlier and more often (p = 0.006). As a result, the total cost for intravenous antibiotics decreased from € 96 500.- to € 81 600.- (p = 0.001). Dosage recommendations (e. g. in impaired organ function) or information on interaction and side effects increased drug -safety. Using the example of antibiotic therapy we showed that pharmaceutical counselling on surgical wards influences various aspects of antibiotic therapy, increases drug safety and reduces cost by having an effect on duration of therapy and timely switch from intravenous to oral preparations.