I'm currently doing the pre op diet and I'm in Australia. My dietician says caffeine is fine as long as it's in moderation. My pre op diet is for 4 weeks the first week was the worst... I had headaches, tummy cramps, fatigue and I was just a grumpy mess. I've been having diet coke every now and then, just because I get sooooo over water. All the best for your op!