Kal Raustiala, writing for Slate:
Quinine powder quickly became critical to the health of the empire. By the 1840s British citizens and soldiers in India were using 700 tons of cinchona bark annually for their protective doses of quinine. Quinine powder kept the troops alive, allowed officials to survive in low-lying and wet regions of India, and ultimately permitted a stable (though surprisingly small) British population to prosper in Britain’s tropical colonies. Quinine was so bitter, though, that British officials stationed in India and other tropical posts took to mixing the powder with soda and sugar. “Tonic water,” of a sort, was born.
Sugar couldn’t mask all the bitterness, though, so it wasn’t long before they started adding gin.