Tetrovaline
From Star Trek : Freedom's Wiki
Tetrovaline is a medical drug used to knock a person unconscious, but which also lowers immune response.
It was carried aboard the Delta Flyer. In 2376, when Crewman Telfer had a dark matter lifeform inside him, he asked to be put under with tetrovaline, but Captain Janeway refused because he needed to keep his immune response up. (VOY: "Good Shepherd")
