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. Poisons secreted by animals are called venom. Nerve gas is a synthetic poison.
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")
K'mpec, the Chancellor of the Klingon Empire, was killed with the poison Veridium Six in 2367. K'mpec stated that the surreptitious use of poison, as opposed to an assassination where the attacker shows his face, was considered a dishonorable act amongst Klingons. (TNG: "Reunion")
Commander Benjamin Sisko was skeptical about the orange bubbling liquid he was offered while in the third shap of the Wadi game Chula and thought that it could be a drug or poison, but ultimately it was the antidote for the gas. (DS9: "Move Along Home")
The Vorta were 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. Afterward, he remarked that their physiology "comes in handy when you're a diplomat". (DS9: "Ties of Blood and Water")
Following an incident in which Seven of Nine suggested individually to Commander Chakotay and Captain Kathryn Janeway that each was involved in a conspiracy against the other, the two met in Janeway's quarters and shared coffee. Chakotay jokingly asked "You didn't poison the coffee, did you?" and Janeway replied "Not any more than I usually do". (VOY: "The Voyager Conspiracy")
When the USS Voyager was shattered into 37 separate timeframes, Commander Chakotay tried to convince Captain Janeway of this by offering to inject her with a hypospray containing a serum that would make her immune to the effects of changing timeframes. She declined the offer, stating that for all she knew, it could be poison. He then grabbed her and held the hypospray to her neck, stating that it was poison and he'd use if the security guard that was there didn't drop his weapon. (VOY: "Shattered")