I used to have coffee and tea while water fasting but not anymore. While coffee helps with the initial headaches and may provide more energy in the following days, the fact that caffeine is a psychoactive drug started to worry me when doing the fasts. Surely, I don't think it causes any damage and by all means, have some if it helps you. But I've noticed it affects my primary goal with fasting: putting my system in a more natural state than before. It took me a while to remove it from my fasting practice.