It's true that quinine was once the principal treatment for malaria, but this is no longer the case. In the UK, the drug watchdogs, MHRA and NICE do not approve the prescribing of quinine. The reasons for their decisions are manyfold, but do include such things as quinine's ability to cause heart rhythm disturbances and no clear evidence that it is of any use for one of it's historical uses, the treatment of leg cramps and restless legs.
All the same, millions of prescriptions for quinine are issued each year in the UK to treat leg cramps etc usually when other treatments have failed.
The inclusion of quinine in foodstuffs is probably one of the biggest uses of quinine today. Historically, it was used to impart bitterness to a product and it's used for the purpose in Indian Tonic Water and Bitter Lemon mixers. In the UK, Indian Tonic Water contains about 90mg of quinine per liter with Bitter Lemon slightly less. So every time you have your G nd T, you're taking in a small amount of quinine. Natural quinine is derived from Cinchona tree bark, also known as The Fever Tree and that's where the premium tonic water called Fever Tree got it's name.