A friend of mine, who is a nurse, recently told me that it is not good (for your kidneys) to drink large amounts of tonic water with quinine added. I do not add any alcohol to the tonic water, so I wanted to find out why this drink might cause health problems.

I hope a health professional does come up and answer this, Preston. I'm 'older generation' and frequently wake with aching legs. My doctor advised me to eat bananas and drink tonic water, so I have an interest in this also.

I hope this helps.
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Thank you for finding those sites, LVLF. I can't speak for Preston, but the key consideration seems to be the 'large quantities' mentioned.

Well, I hope you find what you need. I know many of the volleyball players that go to the bar I used to work at drink tonic and one even takes quinine tablets. As far a drinking large quantities, it's kind of tough to say what is considered too much. I'd think that a couple glasses a day would be safe. I hope your leg cramps get better.

Tonic water contains about 2mg of quinine per ounce.
Quinine has similar side effects to aspirin when taken in excess. Unlike aspirin, overdose can cause blindness which may be permanent. Ocular effects correlate with a blood level of 10mg/mL at 10 hours after ingenstion.
You can read the full report at click here.
Other informative sites are: click here.
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Thanks so much to LVLF and eclairewl for your replies and the links that were provided. (Looks like I'll switch from drinking tonic to drinking seltzer water.) Again, thanks for your help!