Felodesine is a substance that is poisonous to Romulans. Often formed into thin discs approximately one inch in diameter, felodesine was used by members of the Romulan military to commit suicide after capture. A fast acting substance, United Federation of Planets medical science had no known antidote for felodesine.
Admiral Alidar Jarok carried a felodesine chip with him when he decided to defect to the Federation after learning of an apparent new Romulan campaign of aggression against the Federation. After learning that the entire campaign was nothing more than disinformation to expose him as a traitor, Jarok used the felodesine chip to commit suicide a short time later. (TNG episode: "The Defector")