Quinine is an extract of the cinchona plant. It was the original treatment for the malaria parasite, and works both as a preventative and a curative for the disease. It can also be used as an anti-pyretic and analgesic. However, it has largely been replaced by other drugs since the 1940s.
Quinine is the main flavor in tonic water, which was originally sold as a malaria cure, but has become popular as a mixer, particularly with gin, which was originally used to counter the taste of the quinine, which is widely regarded to be one of the most bitter substances on the planet.
In Living The Dream, the patient was allergic to quinine, which House did not determine until he got a new HDTV set which allowed him to see that the liquid the patient was drinking on set may have had fake gin, but had bubbles in it indicating it was real tonic water.