A deadly nerve gas used by the Klingon military, which Leonard McCoy diluted with alcohol to protect the Enterprise crew from the madness caused by interphase space.
Theragen is mentioned in the episode The Tholian Web. The Enterprise is holding position in a region where space is breaking down, waiting to recover Kirk, who is trapped aboard the drifting USS Defiant in another dimension.
The distorted space affects the human nervous system and drives crew members into violent rage, as it had the crew of the Defiant before them. McCoy searches for a counter-agent while arguing with Spock about whether to stay.
His answer is a derivative of theragen. In pure form the gas is lethal, but diluted in alcohol it simply deadens certain nerve inputs to the brain. Scott cheerfully suggests it would go well with Scotch, and the mixture keeps the crew steady until Kirk is recovered.