This question arose from my personal experience - after quitting coffee, I switched to drinking tea ( a lot of tea ). One day there was no tea, and I had a terrible headache, similar to the first days without coffee. A friend of mine suggested there is more caffeine in black tea than in coffee, and suggested green tea.
This made me wonder: how much caffeine is there in the average tea bag, as opposed to two teaspoons of grain coffee ? How about caffeine contents of other tea varieties ( green, white ) ?