The secret police of my fictional country (unoriginally) uses sodium thiopental in medical interrogations, usually in combination with various forms of physical torture.
An antagonist (a courier of smuggled goods) who is detained by them and injected with a thiopental-based serum suddenly dies before the questioning officer arrives. The doctors determine that death occured due to the lethal effects of combining sodium thiopental with a certain substance the courier was injected prior to the mission.
My question is:
Which medical substance or drug is
- lethal (leading to a quick death) in combination with sodium thiopental (or one or more other truth serums), unleashing its effect if it is already present in the body when thiopental is injected,
- stored in the body for at least several hours after initial injection or ingestion, retaining its "latent combinative lethality" during that time and
- has no strong noticeable negative effects without the presence of sodium thiopental.