Some people are very sensitive to quinine and might have serious reactions to the quinine in tonic water, according to The People's Pharmacy. Those reactions can include headaches, rashes, bleeding problems and kidney damage. Other people indicate that the quinine in tonic water can be helpful in treating leg cramps, although this usage is not recommended by medical researchers or the FDA.
Quinine is used as an anti-malaria drug, and in the past was prescribed to people suffering from leg cramps. The FDA has now ruled that it is too toxic to use for purposes other than treating malaria, according to The Peoples Pharmacy. However, the FDA limits the amount of quinine in tonic water to 83 parts per million and says that this amount is considered safe.
Some people may be very sensitive to quinine and may experience reactions to the amount of quinine in tonic water, reports The Peoples Pharmacy. These reactions can include headaches, ringing in the ears, nausea, vision changes or rashes. Potentially fatal reactions may also result from consuming quinine, according to PubMed. These reactions may include bleeding problems, irregular heartbeat or kidney damage.
People who are sensitive to quinine should avoid drinking tonic water. PubMed also recommends that pregnant women and people with kidney problems should avoid ingesting quinine in any form, and cautions that the amount of quinine in commercial beverages can vary significantly.
Although some people claim that tonic water has helped their leg cramps, MedLine Plus cautions that the treatment is not proven to work and can be very dangerous.