Antitoxin is one of three recipes (Poison Extract, Poison Bomb and Antitoxin) you learn from the Night Witch, but she only shows you a picture and you have to figure out what it means.
There are also two recipes (Aphrodisiac, Love Potion) the Day Witch teaches you, but only by giving you a sort of poetic description that you have to figure out.
Not everyone likes it when someone posts the exact recipes here, but there is an old guide that has a lot of recipes in detail, including Poison Extract, Poison Bomb, Antitoxin, Aphrodisiac, and Love Potion.
https://docs.google.com/spreadsheets/d/1kk1nVwdcqxsqkysRpLgMoRE0wKDOmMXYfxKpDsBl...
You need to craft each of these recipes once and then they will appear in your craft menu.