But maybe it sounds too good to be true? Today, I decided to google the whole thing and came up with the following.
1. The quinine is part of the bitter tonic.
2. The drink came about as the early colonialists tried to mask the bitter quinine taste with gin.
3. To prevent malaria one needs to drink the equivalent of 67 liters of GTs per day according to the travel doctor here.
So the answer to my question is unfortunately NO, Gin and Tonics' do not prevent malaria. Well, that is if you consume less than 67 liters a day.
Pic from cafepress.