The quinine that tonic water contains can be effective in treating the symptoms of restless leg syndrome, according to Progressive Health. However, the U.S. Food and Drug Administration banned the use of quinine for this syndrome in 2006, citing its potential to cause lethal effects.
Possible side effects of quinine include low blood sugar, abnormal heart rhythm and electrolyte imbalance, notes Progressive Health. In spite of these side effects, some doctors still suggest the off-label use of quinine to patients with restless leg syndrome, as many do not believe that quinine poses a significant risk. Quinine is a common ingredient in beverages such as tonic water. Most countries set a limit on how much quinine manufacturers can add to their tonic water, minimizing the likelihood of adverse effects.