Tetra-lubisol was a thin, low viscosity, milky fluid in use as a lubricant throughout starships in the late 23rd century. It was quite poisonous to Humans; a dose of as little as a few milliliters was sufficient to cause symptoms, including severe asphyxia and unconsciousness.
In 2266, Lenore Karidian attempted to murder Kevin Riley by poisoning him with this substance. Doctor McCoy was able to save Riley's life, although it was touch and go at first. Despite his beliefs otherwise, he suggested that the contamination could have been as a result of carelessness, given that Riley's drink and the lubricant appeared similar. (TOS: "The Conscience of the King")