I was looking up the meaning of quinine and found this articular.
The quinine in tonic water helps give it a bitter taste. People should not mistake tonic water for a healthful drink, as it may contain sugar and provides no additional nutritional value. Tonic water cannot help a person with leg cramps or restless legs syndrome.

The quinine in tonic water is very diluted. December 12, 2006, the FDA ordered unapproved quinine drug products to be removed from the market and cautioned consumers about the off-label use of quinine to treat leg cramps.