Tonic is a soft drink made from water and Quinine. It's the quinine that gives it that bitter taste.
But what is Quinine?
It is extracted from a small South American shrub, the Cinchona. Originally, it was used against malaria, as a digestive (not the one we know today, of course, the real, the healthy, the one that is good for health!) Or as a cleanser. It is an analgesic and especially a natural antimalarial.
So much for the botanical session of the product.
But then you will say to me: "How did quinine come to be as a basic ingredient of the famous Gin Tonic, or Gin To, or even G&T ?! Well, the quinine being so bitter, very quickly we add sugar, honey or cane sugar. Sugar, cane sugar, cane sugar… you see where I'm going with this? … and hop !! Rum !! Other plants can be used to make a tonic, such as Gentian for example.
Soda became popular in the mid-1800s, and it was Schweppes who first took over Tonic with his Indian Tonic. The brand adds citrus extracts.
Bitterness is a flavor. Perceived by the taste buds of the tongue, but not only. No less than 25 types of receptors detect bitter compounds.