A potent stimulant used by Starfleet doctors in tiny doses; an accidental overdose sends McCoy into the paranoid frenzy that sets The City on the Edge of Forever in motion.
Cordrazine is a strong chemical stimulant carried in Starfleet sickbays and given in very small amounts to revive patients in cardiac or other emergencies. In large doses it causes violent paranoia and delusions.
The drug is introduced in the Original Series episode The City on the Edge of Forever. McCoy uses a couple of drops to bring Sulu round after an accident on the bridge, but a lurch of the ship makes him inject himself with the entire hypospray. Raving, he beams down to the planet below and leaps through the Guardian of Forever, altering Earth's history.
Cordrazine is still in use a century later, and doctors in The Next Generation and its sister series call for it when a patient's life signs are failing.