Lacosamide is one of the third-generation antiseizure drugs that block voltage-gated sodium channels by enhancing slow inactivation. The most common adverse effects of lacosamide include dizziness, headache, nausea, vomiting, diplopia, fatigue, and sedation. Less common side effects include memory impairment, weight gain, rash, and atrioventricular block. In this article, we describe a patient with Lennox-Gastaut syndrome who developed excessive laughing as a rare side effect of lacosamide with complete resolution after discontinuation of the medication. The present case illustrates that excessive laughing may occur as an adverse effect of lacosamide.