Turath

Turath was a poison that was used by the Romulans. It left a tell tale pungent aroma and was considered a foolproof poison as it worked instantaneously. It was rarely used because of the fact that it left an aroma that worked others of the poison. It could be implanted within the mouth of agents who simply needed to bite down and swallow allowing the substance to kill the agent. Typically, it was used for merciful deaths or quiet executions. (TOS novel: Vulcan's Soul Epiphany)