The primary benefit of drinking tonic water is the ingredient quinine. Quinine helps to fight off malaria. This disease comes from the bites of certain mosquitoes.
While maintaining the far-flung British Empire, British soldiers had to work in conditions that involved heavy exposure to mosquitoes, which resulted in the need for quinine. Gin and tonic, one of the most commonly ordered drinks in Great Britain, was invented in the early nineteenth century. A British officer stationed in colonial India discovered that gin helped tonic water go down more agreeably, and a cultural icon was created.
Modern tonic water still has quinine, but the typical formulation is sweeter and more diluted. The purpose now is to help gin get down the throat more easily, so the tables have been turned. Tonic water is one of the few drinks that were once medicines that people now consume just for the purpose of taste.
Quinine does have some side effects that make it questionable to use as a primary medicine, though. For example, thrombocytopenia, or a decrease in platelet count, sometimes leads to external and internal bleeding. Due to side effects, the American government has tightly restricted prescriptions of quinine for medical purposes.