Fever Tree Premium Indian Tonis Water, 16.9 fl oz (500ml) - 4 bottles Quinine is the key ingredient that creates the gentle bitterness in tonic water. Our name was chosen as it is the colloquial name for the cinchona tree, the bark of which produces quinine. After discovering early 17th century references in the British library, Charles and Tim located the only remaining plantation of original Cinchona Ledgeriana trees (known locally as ‘fever trees’) which produce some of the highest quality quinine in the world, located in the Democratic Republic of Congo. For Charles and Tim, there was no other option but to source Fever-Tree’s quinine from this area.