Vancomycin dosing and monitoring recommendations are poorly adhered to in many institutions internationally (1), with concerns of treatment failure and propelling antibiotic resistance. The primary aim of this study was to audit the rate of adherence to American guidelines, with particular interest in loading dose administration. The secondary aims were (1) to determine whether guideline adherence results in therapeutic concentrations across BMI groups and (2) whether this was in turn associated with morbidity and hospital mortality.Data was collected in a single tertiary hospital on all patients who had two or more serum vancomycin concentrations measured.107 patients met the inclusion criteria. Overall, 38.3% of patients were commenced on guideline adherent vancomycin doses. 28.3% of overweight patients received an adherent first dose compared to 51.1% of non-overweight people (difference 23%, 95% CI 4% to 41%, p= 0.024). Overweight patients were more frequently underdosed compared to non-overweight patients (p = 0.039). The frequency and proportion of underdosing increased with BMI. Overweight patients spent a smaller fraction of their course within the therapeutic range, although the difference was not statistically significant (difference 7.7%; 95% CI 4% to 19.4%; p=0.195). The overweight group had longer hospital length of stay (LOS), higher mortality and more treatment failures.Adherence to guideline-based prescription is poor, particularly in overweight patients. Patients who are initially underdosed have less therapeutic vancomycin days, regardless of BMI. Overweight patients have increased hospital LOS, hospital mortality, and treatment failure.