The most serious side effect of quinine use is thrombocytopenia, which is a drop in the blood platelet count, according to LiveScience. This reduction in platelets causes serious bleeding, both internally and externally. The condition also causes permanent kidney damage. Fortunately, the levels of quinine in tonic water are low enough that most people need to drink 20 liters in a day to experience any problems.
LiveScience indicates the original use of tonic water in the 1800s was to supply quinine to individuals with malaria. Mixing quinine with an alcoholic drink made it easier for some malaria patients to get their medication. As time passed, tonic water became sweeter, and bottlers lowered the dosage of quinine. Even with the lower levels of quinine in tonic water, people develop a reaction to the substance in some instances. In some cases, people develop an allergy that allows them to tolerate quinine in tonic water, but causes a serious reaction later in life if they require the substance at prescription strength.
Doctors continue to prescribe quinine to treat malaria, according to Drugs.com. They normally prescribe other antimalarial drugs along with quinine. It kills the parasite that causes malaria; however, doctors do not use it in the prevention of malaria. The Food and Drug Administration banned the off-label prescribing of quinine for leg cramps, according to LiveScience.