Chronexaline is an experimental medical drug created by Starfleet Medical. It protects biomatter from tachyon radiation.
This drug was created sometime around 2404, at least in an alternate timeline, when the Doctor was one of the people working on it. Admiral Kathryn Janeway demanded 500 milligrams of it for a classified mission, which later turned on to be an illegal trip through time to 2378, to ensure the USS Voyager's safe return 16 years earlier than had happened. (VGR: "Endgame").