The drink gains its name from the medicinal effects of this slightly bitter flavoring. The quinine was added to the drink as a prophylactic against malaria, since it was originally intended for consumption in tropical areas of India and Africa where that disease is endemic.
Tonic water originally contained only carbonated water and a large amount of quinine. However, most tonic water today contains a medically insignificant amount of quinine, and is thus used for its flavor only. It is consequently less bitter, and is also usually sweetened. Some manufacturers also produce diet tonic water. In the United States, the Food and Drug Administration limits the quinine content in tonic water to 83 ppm (83 mg per liter if calculated by mass), which is one-half to one-quarter the concentration used in therapeutic tonic.
Tonic water is often used as a drink mixer for cocktails, especially those made with gin (for example, a gin and tonic). Tonic water with lemon or lime flavor added is known as bitter lemon or bitter lime, respectively. Such soft drinks are more popular in Europe than in the United States.
Tonic water will fluoresce under ultraviolet light, due to the presence of quinine. In fact, the sensitivity of quinine to ultraviolet light (UV) is such that it will fluoresce in direct sunlight.