title_body: Does coffee cause dehydration?
upvoted_answer: Though caffeine's action as a diuretic is hotly debated, recently it seems the scientific consensus is that caffeine in normal doses is a weak diuretic in caffeine-naive individuals only (those without a tolerance), and only in doses exceeding 250-300 mg, or 2-3 cups of coffee. To go even further,
  
  
    A profound tolerance to the diuretic and other effects of caffeine
    develops, however, and the actions are much diminished in individuals
    who regularly consume tea or coffee.
  
  
  And from the same review of the literature: 
  
  
    Doses of caffeine equivalent to the amount normally found in standard
    servings of tea, coffee and carbonated soft drinks appear to have no
    diuretic action.
  
  
  The New York Times exposed this myth in this article:
  
  
    Researchers found no significant differences in levels of excreted
    electrolytes or urine volume [between those given caffeine and a placebo].
  
  
  Here is another good resource for answering these questions in a clear, FAQ-style and features a good list of references on the topic.