Even water intake requires moderation: recent media reports have focussed on people who have drunk too much water, which can overwhelm the kidneys and lead to swelling of the brain. Water intoxication can cause symptoms of headache, dizziness, nausea, confusion and, in extreme cases, can lead to coma and death, although this is rare. Certain drugs, including illegal ones such as ecstasy, can trigger drinking excessive amounts of water. It can also be caused by health conditions that affect water balance, such as kidney failure, liver cirrhosis, and uncontrolled diabetes. Drinking excessive amounts of water can also have a psychological cause when it is known as psychogenic polydipsia.
Most people will come to no harm if they stick to drinking two to three litres of fluid per day.
http://www.ivillage.co.uk/health/hlive/eat/articles/0,,181168_588681-2,00.html
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