Aim:To compare the adverse effect profile in the 1-hour postadministration period for a 3-minute gentamicin 40 mg/mL bolus with a 30-minute gentamicin 1 mg/mL infusion. Method: Australian guidelines recommend that IV gentamicin doses greater than 240 mg need to be diluted and infused over 20 to 120 minutes, while lower doses can be administered as a bolus using the undiluted injection solution (40 mg/mL) over 2 to 5 minutes. Patients receiving at least 2 gentamicin doses of 240 mg or greater were administered an undiluted IV bolus (40 mg/mL) over 3 minutes on one occasion and a 1 mg/mL infusion over 30 minutes on another in a randomised crossover design. Patients were assessed for pain, swelling and redness at the injection site, tinnitus, dizziness, headache and nausea at 0, 15, 30 and 60 minutes after administration of the gentamicin dose. Results: 47 patients completed the study. Mean administration rates in the bolus and infusion arms were 105 + 27 mg/min and 12 + 8.1 mg/min, respectively. During the bolus phase, more patients complained of increased pain at the injection site immediately after completion of the bolus dose (34% vs 9%, p < 0.01), but this effect had dissipated at the 15-minute assessment (4% vs 4%). There were no differences in any other parameters at any of the assessment times. Conclusion: Bolus gentamicin of up to 480 mg (undiluted 40 mg/mL injection) administered over 3 minutes can be used as an alternative to a diluted gentamicin infusion, but may cause transient IV site pain. J Pharm Pract Res 2012; 42: 200-3.