The real mechanism of VPA-induced hyperammonemia remains unknown.

Valproic acid (VPA) is a well-tolerated and effective agent for the treatment of epilepsy, bipolar disorder, and schizoaffective disorder. Several case reports have indicated that VPA may induce serious symptomatic hyperammonemia. Based on analysis of susceptible patients, several possible mechanisms and risk factors have been proposed to identify the patients at risk. Nevertheless, we report the case of a schizoaffective patient who developed severe hyperammonemia occurring after brief exposure to VPA, despite the absence of any known risk factors. Until now, early recognition of the signs and symptoms of hyperammonemia is crucial to managing this unusual adverse reaction.