Tonic water is traditionally a quinine based bitter drink. Quinine comes from the bark of the South American cinchona tree - the word quinine is from the Quechua (Inca) word for that bark. The tonic of the 1600s (quinine, sugar and water) bore very little resemblance to what we now know as tonic water, and was in fact used originally as an anti-malarial treatment. The quinine was mixed with sweetened water to make it palatable, but British colonials still found it too bitter and mixed it with gin, thus creating the gin and tonic.
Many astute bartenders have realised that widely available tonic water, typically made from high-fructose corn syrup, carbonated water and synthetic quinine, can ruin a quality gin and tonic. This is why you can now buy tonic syrups with combinations of real quinine, fruit, herbs and natural sweeteners. They are essentially, quality tonic water, minus the water. The magic of tonic syrups is that they give you complete control over the flavour. You pour in the syrup and then add soda water to dilute to taste.
Need help choosing? Here is our Tonic Guide.