A tranquilizer gun was a form of non-lethal weapon which could incapacitate a target for a short period of time. One or more shots from this weapon could render the victim unconscious for a time. Once the tranquilizer wore off, the victim regained consciousness.
Jack Bauer used a tranquilizer gun to incapacitate guards while kidnapping Lee Jong from the Chinese consulate, who was later questioned and gave up three locations that he knew Habib Marwan would be using previous to and throughout the course of Day 4.
A CTU Charlie Team field agent used a sniper rifle tranquilizer gun to bring down Dmitri Gredenko during the sting operation with Brady Hauser. He fired one dart into his neck to knock him unconscious, so that Jack Bauer could later question him.