Antitoxin is a written note found in The Brigmore Witches.
The antitoxin formula needs the following:
Oxrush Flower (Buy more from Jerome in Drapers Ward.)
Bull Rat Fetus (one in the dry storage room)
Metallic Salts (some on the manufacturing floor)
It can be found in a secret wall compartment near an audiograph player in Nurse Trimble's lab during the mission The Dead Eels.