Minecraft wiki entry for Brewing, associated text: Effect potions are primarily created by adding an effect ingredient to an awkward potion, which is created by adding nether wart to a water bottle. Certain effects require a potion to be corrupted by a fermented spider eye. The potion of weakness can additionally be created by simply adding a fermented spider eye to a water bottle — and it is the only potion that can be brewed without nether wart.