How alchemy works in The Witcher 3
Alchemy in The Witcher 3 splits four ways: potions and decoctions Geralt drinks, oils he coats a blade with, and bombs he throws. Potions, oils and bombs all refill for free when he meditates with strong alcohol in the satchel, so the running cost is a bottle of Alcohest rather than the ingredients.
Toxicity is the real limit on Witcher 3 alchemy. Every potion pushes it up, decoctions push it high and hold it there for the whole duration, and crossing the bar drains Vitality instead of helping. Treating a decoction as a slot you commit to, rather than something you drink mid-fight, is the quickest way to plan around it.
Witcher 3 oils, bombs and what to brew first
Oils are the category worth carrying in full. Each one is made for a single monster class, and every creature in the Witcher 3 bestiary lists the class it belongs to, so the right oil follows from the entry instead of from guesswork.
Recipes live on the Witcher 3 alchemy formulas page and the things they consume are on the alchemy ingredients page. Superior tiers take the enhanced version as an ingredient, so each ladder has to be climbed in order.