Tonic water gets its distinct taste from quinine, a bitter compound that is extracted from the bark of the cinchona tree. Along with quinine, Schweppes Tonic Water contains high-fructose corn syrup, citric acid, and natural flavors to give it its refreshing taste. It is often served with a wedge of lime or lemon to balance out the bitterness of the quinine. Schweppes Tonic Water is a staple in most bars and is a popular mixer for a variety of cocktails.