Side effects and risks
Quinine in tonic water is diluted enough that serious side effects are unlikely. If you do have a reaction, it may include:
nausea
stomach cramps
diarrhea
vomiting
ringing in the ears
confusion
nervousness
However, these are more common side effects for quinine taken as a medication. Among the most serious potential side effects associated with quinine are:
bleeding problems
kidney damage
abnormal heartbeat
severe allergic reaction
Keep in mind that these reactions are primarily linked to quinine, the medication. You would have to drink about two liters of tonic water a day to consume a day's dose of quinine in pill form.
Who should avoid quinine?
If you've had a bad reaction to tonic water or quinine in the past, you should not try it again. You may also be advised against taking quinine or drinking tonic water if you:
have an abnormal heart rhythm, especially a prolonged QT interval
have low blood sugar (because quinine can cause your blood sugar to drop)
are pregnant
have kidney or liver disease
are taking medications, such as blood thinners, antidepressants, antibiotics, antacids, and statins (these medications may not preclude you from taking quinine or drinking tonic water, but you should tell your doctor about these and any other medications you take if you're prescribed quinine)