Recent research demonstrates a potential association between metformin use and reduced sporadic vestibular schwannoma (VS) growth in patients undergoing conservative observation. The current study was designed to elucidate the effect of metformin on tumor growth in sporadic VS using volumetric analyses. Retrospective cohort. Tertiary referral center. Patients with sporadic VS who elected initial conservative treatment with at least two serial magnetic resonance imaging (MRI) scans were included. Metformin use among patients with observed sporadic VS. Tumor growth, defined as an increase in volume of at least 20% from the initial MRI. A total of 361 patients were evaluated. Thirty-four patients (9%) had a diagnosis of diabetes at baseline. Nineteen patients (5%) were taking metformin at the time of the initial MRI. Metformin use was not significantly associated with a reduced risk of volumetric tumor growth in a univariable analysis in all patients undergoing observation for VS (hazard ratio [HR] 0.75; 95% confidence intervals [CI] 0.40-1.42; p = 0.38) or within the diabetic subset (HR 0.79; 95% CI 0.34-1.83; p = 0.58). Additionally, diabetes status, insulin dependence, hemoglobin A1c value, and metformin dose were not significantly associated with volumetric tumor growth. Despite promising initial results in several previous studies, our data suggest that metformin use does not significantly reduce the risk of volumetric tumor growth in sporadic VS.