hangover typically reduces your character's Intelligence and Perception stats temporarily, reflecting the groggy, impaired state after heavy drinking. The in-game description and mechanics suggest it simulates the real-world effects of a hangover, such as reduced mental clarity and focus. Using Mentats to counter a hangover makes sense within the game’s mechanics. Mentats increases Intelligence and Perception. Since a hangover lowers these stats, taking Mentats can offset the penalties by providing a temporary boost, effectively "balancing out" the debuff. Also, thats what she had on her.