I agree with Snowman's answer. The other thing to remember is if you're exercising or working in a very hot environment, you can drink gallons of water and still find yourself with a medical emergency. Replacing water is great, but if you're sweating a lot, you also have to replace lost electrolytes, minerals, etc. In the situations I mentioned, drinking water is a must, but you should also be drinking some type of sport drink. I won't make any recommendations because everyone has different opinions about which are best. Talking to a doctor or sports trainer will be your best bet.