There is research that suggests that caffeine will negate creatine's effects. It will not affect creatine's effects in lifting weights but rather it will diminish creatine's effects in running and jumping. Caffeine slows down Ca++ uptake (The faster it is taken up the quicker your sarcomeres relax and then contract again - speed in sense). If you are taking creatine for sports such as rugby, basketball etc.. I would not take caffeine, but if you are taking it for lifting weights (strength, cell volumizing) then caffeine is ok