Lourkain was a type of pharmaceutical compound formulated by the Klingon civilization as a type of truth serum. Klingon doctors typically administered lourkain to injured patients in order to interrogate them while they recovered. The drug was made from an alkaloid base that was quite similar to a naturally-occurring compund in the persaba fruit from the planet Peneli.
In 2270, Doctor Eknaar of the IKS Klolode II used dosages of lourkain on victims of an earthquake on Sherman's Planet in order to prepare them for questioning. The only two survivors of the agricultural research outpost were conditioned to respond negatively to lourkain by Starfleet as a precaution against Klingon espionage. The severely injured communications officer, Jones, died shortly after being administered lourkain. The other survivor, Specialist Jean Czerny, suffered a severe allergic reaction and had to be stabilized by Eknaar. The conditioned allergy continued to affect Czerny in her travels throughout the Klingon Empire, as she also suffered ill effects after consuming persaba. (TOS novel: Pawns and Symbols)