Veridium Six is a chemical substance that is used as a poison.

Sometime in 2366, Klingon High Council member Duras began using Veridium Six to poison Chancellor of the Klingon High Council K'mpec in the hopes that he would be able to succeed K'mpec as Chancellor. This was despite the fact that killing in this manner was dishonorable as it allowed a Klingon to kill without showing his or her face. (TNG episode: "Reunion")