A milky and highly poisonous lubricant used aboard starships, slipped into Kevin Riley's drink in an attempt to silence one of the last witnesses to Kodos the Executioner.
Tetralubisol appears in the Original Series episode The Conscience of the King. It is a volatile lubricant carried in the Enterprise's stores, white enough to pass unnoticed in a glass of milk.
Lieutenant Kevin Riley, one of the few survivors able to identify Kodos of Tarsus IV, is found collapsed in engineering after someone sprays the substance into his drink. McCoy saves him, and the poisoning convinces Kirk and Spock that the deaths of other witnesses were no coincidence.
The poisoner proves to be Lenore Karidian, daughter of the actor Anton Karidian, who has been murdering everyone who could expose her father's past.