Tonic water, a carbonated beverage, is primarily flavored with quinine, a compound derived from the bark of the cinchona tree. This ingredient gives tonic water its distinctive bitter taste. Often, it also contains other flavorings and sweeteners, such as high fructose corn syrup or sugar, to balance out the bitterness. The effervescence and unique flavor profile make tonic water a versatile mixer in cocktails.

In the context of alcohol, tonic water is frequently paired with gin to create the classic gin and tonic. Its bitterness complements the botanical notes of gin, enhancing the overall drinking experience. Tonic water is also used in other cocktails, such as vodka tonics and non-alcoholic mocktails, offering a refreshing and slightly bitter edge that can elevate various drink recipes.