Although coffee is a diuretic, meaning it will make you go pee more, it does not appear to be a likely cause of dehydration. I know I am probably making more friends than enemies with that statement but just to clarify, drinking coffee is not the same as drinking water.
Drinking large quantities of coffee can lead to serious side effects. For the moment, it appears to be a safe option in moderation (1-3 cups per day) and it will not cause dehydration according to current studies.
