Found this info from a quick google search, first hit
http://www.schizophrenia.com/sznews/...es/002417.html had this at the bottom of the page
"This is called hyponatremia and is common when symptoms are untreated or under-treated. To cause hyponatremia, the person has to drink incredible quantities of water, because even with large amounts of water, the body balances electrolytes and remains healthy. "
Although this guy was suffering from Schizophrenia and was on medication