 In case of occurrence of a generalized tonic-clonic seizure, patients will be randomized 1:1 to receive intravenous naloxone 0.4 mg or placebo. Placebo will be isotonic sodium chloride which preparation and packaging will be centralized to ensure its indistinguishability from naloxone. Randomization will be centralized and stratified by centre. The evolution from a partial seizure to a GTCS being gradual, and the total duration of the seizure ranging from 2 to 3 minutes, the injection will be prepared during the course of the seizure. Given the assumptions about the role of endogenous opioids release in the spontaneous termination of seizures, naloxone will be administrated immediately after the end of the GTCS and not before. 