There are two tonics that the Kid encounters in his travels.
Health TonicHealth tonics can be consumed to regain health. At the beginning of the game, they only refill about half of the health bar. Equipping Bastion Bourbon in the Distillery increases the potency of health tonics and allows the Kid to carry up to 5.
Black TonicBlack tonics can be consumed one by one to activate Secret Skills. At the beginning of the game, the Kid can only carry 3. Equipping Black Rye allows up to 5 black tonics to be carries at a time.