I ticked Other for 'what do you put in your drinks'. I don't have anything in tea or coffee, for preference. Sometimes it accidentally comes with milk, no big deal. (the dairy industry is environmentally unsound though) I don't know whether the fats or protein in milk would make a difference to the way caffeine is absorbed.
I forgot to tick Chocolate (dark, for preference, with a minimum of adjuvants) but I think compared to the three or four cups of black tea and the cup of black coffee daily, the caffeine in chocolate (and chocolate derivatives) is negligible.
One time I did spontaneously give up tea and coffee for a week or so, when I was having trouble sleeping. It seemed to have little effect. More than one large cup of plunger coffee a day makes me jittery and a little hyper though. And sometimes, as others have reported, the taste and smell of tea suddenly becomes unpleasant if I have too much.