Poisons are toxicological substances that adversely affect living tissue, with effects ranging from slight discomfort to death. They can be natural or synthetic in origin. Poison secreted by animals is called venom. Nerve gases are synthetic poisons.
The venom of the mugato and the alkaloids of the Borgia plant are examples of natural poisons. Both are capable of killing an adult humanoid. (TOS: "A Private Little War", "The Man Trap") Artificial poisons include Felodesine chips and Veridium Six. (TNG: "The Defector", "Reunion")
On Dimorus, the indigenous rodent creatures use deadly poisoned darts as weapons. (TOS: "Where No Man Has Gone Before")
Sevrin died when he ate a poisonous fruit on Eden. (TOS: "The Way to Eden")
On Ligon II, the dueling weapons are coated with an alkaloid-based poison. As a result, a person could be killed instantaneously by a grazing wound. (TNG: "Code of Honor")
Poisons were the favored method of assassination of the Flaxian Retaya. (DS9: "Improbable Cause")
The Vorta are genetically engineered to be immune to most types of poison. In 2373, Weyoun drank a glass of kanar containing enough voraxna poison to kill a dozen Cardassians. Afterwards, he remarked that their physiology "comes in handy when you're a diplomat". (DS9: "Ties of Blood and Water")