If you drink regular tonic water, be aware that this carbonated cocktail mixer has almost as much sugar as soda: 32 grams (8 teaspoons) and 130 calories per 12-ounce can (compared to 39 grams in a cola, for instance). Quinine, derived from the bark of the chinchona tree, gives tonic water its bitter flavor, but you may notice the beverage’s (often cloying) sweetness. A zero-calorie alternative is seltzer or club soda—or else diet tonic water.
By the way, quinine used to be given at much higher doses to treat malaria. The dose in a glass or two of tonic water is rarely a problem, but some reports have linked it to a drop in platelet count (thrombocytopenia).
Also see Alcohol in Cocktails.