Answer:
Adults should drink 2.5litres every day
However, there is a different story emerging, especially amongst people who exercise. According to researchers almost as many exercisers are putting their health at risk by over-consuming water as drinking too little.
Drinking water at every opportunity can cause serious problems, such as hyponatraemia or water intoxication. As the water content of the blood increases, the salt content is diluted. Consequently the amount of salt available to body tissues decreases, which can lead to problems with brain, heart and muscle function.
Initial symptoms of over-hydration include dizziness, nausea, apathy and confusion. However these symptoms are also associated with dehydration - so it's important to be aware of how much you are drinking.