Quinine is a drug used to treat malaria. It has been used off-label to treat leg cramps, but the side effects outweigh the benefits in this case. Modern tonic water contains much less quinine than is necessary for any medical treatment.
Quinine is found in cinchona bark and was used by native South Americans to treat malaria. Europeans first encountered the drug in the mid-seventeenth century.
Quinine was originally added to tonic water as an anti-malarial treatment. According to legend, gin and tonics were born as a way to mask quinine's bitterness.
Both natural and synthetic quinine are still used today to treat strains of malaria that have grown resistant to other safer drugs.
The FDA has not approved quinine as a treatment for leg cramps. It has dangerous side effects, including blood clotting and death, that may be an acceptable risk for life-threatening disease but not for less serious conditions.
Modern Tonic Water
Today, tonic water only contains twenty milligrams of quinine per six ounces. It would take up to five gallons of tonic water a day to treat malaria.