"Tonic water (or Indian tonic water) is a carbonated soft drink flavoured with quinine, which gives it a distinctively bitter taste. The drink has garnered its name from the medicinal effects of this bitter flavouring. The quinine was added to the drink as a prophylactic against malaria, since it was originally intended for consumption in tropical areas of South Asia and Africa where that disease is endemic. The mixed drink gin and tonic originated in British colonial India when the British population would mix their medicinal quinine tonic with gin to make it more palatable. Medicinal 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 flavour only. It is consequently less bitter, and is also usually sweetened. Sweet tonic water may contain corn syrup or sugar, making it less bitter. Some manufacturers also produce diet tonic water which may contain artificial sweeteners. Traditional style tonic water with little more than quinine and carbonated water is less common but may be preferred by those who desire the bitter taste. In the United States, the Food and Drug Administration limits the quinine content in tonic water to 83 ppm (83 mg per litre if calculated by mass), which is 0.25% to 0.50% of the concentration used in therapeutic tonic." Source and further information: http://en.wikipedia.org/wiki/Tonic_water