Witchers are famous for brewing potions that give them supernatural abilities in combat. To brew them, they harvest alchemy ingredients from roadside shrubs and from the corpses of monsters they kill. Each ingredient contains one of the following basic substances: aether, caelum, fulgur, hydragenum, quebrith, rebis, sol, vermilion or vitriol. Alchemy formulae describe the proportions of these substances which must be mixed to obtain the desired potion. To make a potion while meditating, the witcher must have both a formula and the ingredients it requires.
In addition to potions, witchers can also prepare oils that they apply to blades to poison opponents or irritate their wounds, and bombs which can be cast to wound or kill many foes at once.
Potions have a delayed effect, so they should be consumed before a fight, while meditating. Oils can only be applied to sword blades in the Inventory panel. Bombs should also be prepared in advance and placed in the "Pocket" slots.