The caffeine in coffee or tea (or soda) inhibits antidiuretic hormone (ADH), the effect of which is to promote water loss by the kidneys. This can lead to dehydration. However, those that ingest caffeine containing substances frequently do build up some degree of tolerance (and for the caffeine addicts, going without can lead to low blood pressure, headaches, irritability, difficulty concentrating, among other symptoms of caffeine withdrawal)