Tonic water is a quinine-based bitter-sweet soft drink / mixer. Premium brands include Q Tonic and Fever Tree. Mainstream brands tend to be sweeter, although the Whole Foods 365 brand may than others.
A tonic syrup may be made and combined à la minute with seltzer or soda water, allowing control of the intensity and flavors of the tonic water, as well as the sweetness.
Tonic Syrup Recipes include: