Coffee's actually quite harmless and unless the caffeine bothers you, you can drink all you want.
It's a myth that coffee/caffeine dehydrates you.
According to International Journal of Sport Nutrition and Exercise Metabolism, researcher Lawrence E. Armstrong, a professor of exercise and environmental physiology at the University of Connecticut caffeine is no more a diuretic than water.
Quoting Dr. Armstrong:
“While there have been several studies done, there is no evidence that exercise, when combined with the consumption of caffeine or caffeinated beverages, will result in chronic dehydration, and this is contrary to the advice of most exercise physiologists, physicians and dietitians,” explains Armstrong, who has conducted fluid balance research since 1980.
Supporting Armstrong’s findings is an American Journal of Physiology August 2002 review of the literature on hydration by Dr. Heinz Valtin that cites a University of Nebraska study, which found that caffeinated beverages may be counted toward the daily fluid total.
Other findings of the study:
There is no evidence that consumption of caffeinated beverages causes a fluid-electrolyte imbalance that is detrimental to health or exercise performance.
Caffeine consumption causes a very mild diuresis that is about the same as that of water (water, when consumed in large volume, increases urine output). ***
So, drink up!
***References:
Journals - Human Kinetics
Caffeine and Dehydration: Myth or Fact? (NewsBite)