Potion brewing in Minecraft: every recipe in one place
The exception is the Fermented Spider Eye:

The Spider Eye is dropped by Spiders, cave spiders and witches. It is poisonous to eat, but it is a key brewing ingredient: it gives potions of poison, and fermented, it reverses other potions.
It cannot be crafted; it can be found in chests.
Eating a spider eye restores two hunger points but poisons you for a few seconds, so it is never worth it unless you are starving. Brewed into an awkward potion, it makes a Potion of Poison, a classic for splash potions against groups of mobs.
Combined with sugar and a brown mushroom, a spider eye becomes a Fermented Spider Eye. That ingredient corrupts potions: it turns swiftness into slowness, healing into harming, night vision into invisibility, and creates the Potion of Weakness from a water bottle.
Spider eyes are also the favorite food of Armadillos: holding one makes them follow you, and feeding two lets them breed, which is how you farm armadillo scutes for wolf armor.
This item has no crafting recipe.
The exception is the Fermented Spider Eye:
In a Brewing Stand fuelled with Blaze Powder, add a Fermented Spider Eye to a Water Bottle to get a Potion of Weakness (no Nether Wart needed), then add Gunpowder to…
It can be found as loot in Desert Pyramid. It is dropped by Cave Spider, Spider and Witch.
It brews poison potions, becomes the fermented spider eye and breeds armadillos. It is an ingredient in one recipe: Fermented Spider Eye. Brewing: it turns potions into Mundane Potion and Potion of Poison. Animals that eat it: Armadillo.
2 hunger points (1 drumsticks) and 3.2 saturation. Effects: Poison (level I · 5 s).
A Balanced Diet (Eat everything that is edible, even if it's not good for you).