Data regarding lacosamide treatment as an adjunctive therapy in patients representative of a focal-onset epilepsy population including those with and without intellectual/developmental disorders (IDDs) are limited. To evaluate the retention rates of lacosamide in focal-onset epilepsy patients with and without IDD. We retrospectively reviewed all consecutive electronic and paper medical records of patients diagnosed with focal-onset epilepsy who were treated with lacosamide in two tertiary epilepsy centers. One hundred and thirty-six patients who met the inclusion criteria were studied. Number of patients with IDD was 46 (33.8%). Median lacosamide dose was 300 mg/day. A total of 39 patients (28.7%) experienced side effects, and 22 of them (16.2%) discontinued lacosamide. The 1-, 2-, and 3-year retention rates of lacosamide in patients with IDD were 68%, 62%, and 53%, respectively. Kaplan-Meier survival analysis showed that the retention rates were significantly lower in patients with IDD when compared to patients without IDD ( =0.04). Cox regression analysis showed that concomitant use of sodium channel blocker antiepileptic drugs (AEDs) was the only independent predictor of retention rate of lacosamide treatment ( =0.03). In the subgroup of patients with IDD, the analysis was performed again and the number of background AEDs was the only predictor for the retention rate of lacosamide ( =0.04). When compared to patients without IDD, retention rates of lacosamide adjunctive therapy were lower in patients with IDD. However, these rates were higher than the rates suggested with previously registered AEDs including lamotrigine, levetiracetam, and topiramate. Therefore, irrespective of having comorbid IDD, we might suggest that lacosamide is a well-retained drug with a high efficacy profile in patients with focal-onset epilepsy.