Quinine should not be prescribed or taken to treat or prevent nighttime muscle cramps or restless legs syndrome because it can cause serious or even fatal adverse effects. Formerly a commonly used treatment for leg cramps, prescription quinine sulfate is now FDA-approved only for a type of malaria.
The FDA took over-the-counter quinine tablets off the market years ago and has since warned about off-label use of prescription quinine because of side effects ranging from vertigo, vision disturbances, and ringing in the ears to severe reduction of platelets in the blood, kidney damage, cardiac arrest, and death.
Now an observational study of 175,000 people (average age 70) in the U.K., published in the Journal of the American Medical Association, has found that long-term use of quinine for cramps was associated with increased mortality rates; higher doses were linked to greatest risk.
What about tonic water? Leading brands usually contain little or no quinine, though some do contain significant amounts. Even with quinine, you’d have to drink at least a quart or two of tonic water a day to get the amounts that people in the study were taking (more than 100 milligrams a day).
Also see Dealing With Restless Legs.