Theoretically, by drinking small amounts of truth serum over a long period of time, you could build up an immunity to it. Therefore, drinking a whole lot of it all at once should work even better.
(In-game plural: vials of interrogative elixir)
Playing Bastille Battalion with Truth Serum in the Moat
|You drink the vial of interrogative elixir... or did you?|
|TOP 10 interrogative elixir collections|
6. Txranger - 55 | 7. Poncho the Sane - 32 | 8. Nikademus - 31 | 9. lupus4 - 25 | 10. MysticalMolly - 21
Collection data courtesy of ePeterso2 and Jicken Wings