Potions are one-time-use items that can only be used in combat, with the exception of Fruit Juice, Entropic Brew, and Blood Potion, which can be used in most scenarios. You can typically hold up to 3 potions, or 2 on ascension levels 11 and higher. Potion Belt increases the number you can hold by 2 (up to 5, or 4 on ascension 11+). Using a potion does not require Energy and does not count as playing a card.
When a potion is offered, rarity chances are: 65% Common, 25% Uncommon, 10% Rare.
Obtaining potions. Potions sometimes drop after combat. The default drop chance is 40%, reset each Act; if one drops it decreases by 10%, if not it increases by 10%. White Beast Statue guarantees a potion drop after combat. You can buy potions at the Merchant. After reaching the Act 1 boss, Neow may offer 3 potions. Some events offer potions. Lab offers 3 random potions. The Woman in Blue: 1 random potion for 20 gold, 2 for 30, 3 for 40. Knowing Skull: random potion for 10% of max HP (minimum 6). The card Alchemize creates 1 random potion (other than Fruit Juice). Cauldron offers 5 random potions. Tiny House offers 1. Entropic Brew fills all empty potion slots with random potions (cannot generate Fruit Juice in combat). Sozu prevents obtaining any more potions (including from Entropic Brew), but you can still use ones you have.
Other interactions. We Meet Again! lets you give Ranwid 1 random potion for a Relic. Toy Ornithopter heals 5 HP whenever you use a potion. Sacred Bark doubles potion effectiveness except: Ambrosia, Blessing of the Forge, Entropic Brew, Elixir, Gambler's Brew, Smoke Bomb, Stance Potion.