Tonic water (or Indian tonic water) is a carbonated soft drink in which quinine is dissolved. Originally used as a prophylactic against malaria, tonic water usually now has a significantly lower quinine content and is consumed for its distinctive bitter flavor, though it is nowadays also often sweetened. It is often used in mixed drinks, particularly in gin and tonic.
In early 19th century India and other tropical posts of the British Empire, medicinal quinine was recommended to British officials and soldiers, where it was mixed with soda and sugar to mask its bitter taste, creating tonic water.
The first commercial tonic water was produced in 1858. The mixed drink gin and tonic also originated in British colonial India, when the British mixed their medicinal quinine tonic with gin and other ingredients to make the bitter medicine more palatable. Soldiers in India were already given a gin ration, and the sweet concoction made sense.
Medicinal tonic water originally contained only carbonated water and a large amount of quinine; most modern tonic waters contain comparatively less quinine, and are often enhanced by citrus flavors. As a result of the lower quinine content, tonic water is less bitter, and is also usually sweetened, often with the addition of high-fructose corn syrup or sugar. Some manufacturers also produce diet (or "slimline") tonic water, which may contain artificial sweeteners such as aspartame. 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 flavor.
In the United States, the US Food and Drug Administration (FDA) limits the quinine content in tonic water to 83 ppm (83 mg per liter if calculated by volume), while the daily therapeutic dose of quinine is in the range of 500–1000 mg, and 10 mg/kg every eight hours for effective malaria prevention (2100 mg daily for a 70 kilograms (150 lb) adult). It is often recommended as a relief for leg cramps, but medical research suggests some care is needed in monitoring doses. Because of quinine's risks, the FDA cautions consumers against using "off-label" quinine drugs to treat leg cramps.
Tonic water is often used as a drink mixer for cocktails, especially gin and tonic. Vodka tonic is also popular. Tonic water with lemon or lime juice added is known as bitter lemon or bitter lime, respectively.
The quinine in tonic water will fluoresce under ultraviolet light. In fact, the sensitivity of quinine to ultraviolet light is such that it will appear visibly fluorescent in direct sunlight against a dark background.
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