Results: Ninety-five patients were included in the study. Of these, 55 (58.9%) showed more than 50% seizure reduction. There was no significant difference in efficacy between the monotherapy (n=38) and adjunctive therapy (n=57) groups. Twenty-nine patients (30.5%) experienced somnolence, and nine patients (9.5%) discontinued LEV. In total, nine patients (23.7%) treated with LEV as monotherapy and 20 patients (35.1%) treated with LEV as adjunctive therapy experienced somnolence, with no significant difference between the two groups. Regarding concomitant antiepileptic drug use, only valproic acid (VPA) significantly induced somnolence. Multiple logistic regression analysis showed that LEV as adjunctive therapy to VPA was a risk factor for somnolence (odds ratio, 3.95; 95% confidence interval, 1.43-11.36; p=0.0079).