Tonic water (or Indian tonic water) is a carbonated soft drink, in which quinine is dissolved.Originally used as a prophylactic against malaria, tonic water usually now has a significantly lower quinine content and is consumed for its distinctive bitter flavour.It is often used in mixed drinks, particularly in gin and tonic.onic water is often used as a drink mixer for cocktails, especially those made with gin or vodka (for example, a gin and tonic). Tonic water with lemon or lime flavour added is known as bitter lemon or bitter lime, respectively.