An experimental drug of the early 25th century that shields biological matter from tachyon radiation, taken by Admiral Kathryn Janeway before her unauthorised journey back in time in Endgame.
Chronexaline is a medication under trial at Starfleet Medical in the alternative future shown in the Voyager finale Endgame. Its purpose is to protect living tissue from the tachyon radiation associated with time travel.
In 2404 Admiral Janeway visits the Doctor, who is by then a respected physician, and asks for a supply well beyond what any legitimate project would need. He guesses that she is planning something she has not told him. The request is one step in an elaborate scheme that also involves obtaining a chrono-deflector from the Klingon Korath and borrowing a shuttle fitted with ablative armour and transphasic torpedoes.
Dosed with the drug, the admiral opens a temporal rift and emerges beside Voyager in the Delta Quadrant in 2378, where she persuades her younger self to use a Borg transwarp hub to bring the crew home sixteen years early.