Water Intoxication
Rapid intake of too much water floods the inside of cells when sodium is depleted. This sodium depletion, called hyponatremia, can rupture the cells, either from extra pressure on the cells from without, or from pressure within flooded cells. As cells rupture in various parts of the body, certain symptoms emerge, and usually quickly.
What kind of symptoms? Well, things like vomiting, headaches, confusion and disorientation. If things progress this can lead to more serious things like seizures and coma. Untreated cases have resulted in death. (Like this recent case, or these other sad cases, for example).
Of course these are extreme situations of water intoxication. Generally speaking, we hit the danger zone when we take in more water than what our kidneys can process in a hour. And for someone with extremely healthy kidneys that is about 30 oz. of water in a hour (please don’t drink that much!). Drinking too much water in rapid succession can prove fatal and should definitely be avoided.