You can get Antitoxin recipe after you talk with the witch (daytime) and ask her to teach you. She will show you a picture of ingredients and outcomes which you must solve this riddles to get a recipe.
By the way, here a recipe.
1st slot: Poison extract (the first one in a picture which make from 2 pulp (green) and 2 mushrooms)
2nd slot: Pulp (red)
3rd slot: Angle leaf
4th slot: Pulp (red)
5th slot: leave it blank
And I suggest you to brew it for sparing, this item is so important to get a good ending in some events.
