Water overdose, also known as water intoxication, refers to a potentially life-threatening dilution of the blood stream. Though water is one of the key elements of life and contributes approximately 2/3 of the total content of the human body, too much consumed in too short of a period of time may over-dilute the concentration of electrolytes and nutrients in the blood stream. This condition is reflected in the medical name "hyponatremia," which translates to an abnormally low quantity of sodium in the blood. It should be noted that water intoxication is a severe version of hyponatremia, however, and that the medical term can also refer to non-lethally-low levels of sodium in the blood steam, anything below 0.4 ounces per gallon (135 millimoles per liter).
When the rate at which water is consumed exceeds the rate at which water is used by the body or removed by the kidneys, water overdose occurs. A normal, healthy kidney will process 0.21 to 0.26 gallon (800 to 1,000 milliliters) of water every hour. During times of heavy physical activity or stress, however, the pituitary gland will excrete increased amounts of vasopressin, a hormone that can cut the removal of water by the kidneys by 90%. This means that even if sweating fairly heavily, a person may actually be able to consume the amount of water their kidneys would normally process in an hour and still have net a gain of fluid.
The greatest danger to the body from water overdose comes as a result of osmosis, the natural movement of a liquid from an area of higher concentration to an area of lower concentration. When the balance of water to sodium in the bloodstream is off, the body uses osmosis to correct it by exchanging water with the body's cells through the cell walls. In the case of water overdose, this process results in the cells becoming bloated with water. For most cells, the walls simply stretch and there are no major issues, but cells in the brain have no room to expand. This translates to increased pressure on the brain and possible death.
Symptoms of water overdose include headaches, drowsiness or fatigue, irritability, and confusion. Additionally, a victim may experience unexplained numbness of limbs, cramps, general muscular weakness, and even have difficulty breathing. Further symptoms depend on the parts of the brain being pressed, but can effect the heart and pulse as well as motor functions. As with any condition, the best plan is to be aware of one's own behavior and condition and how they relate to symptoms.
Many of the symptoms of water overdose are shared by conditions such as heat stroke and dehydration, so assuming the symptoms indicate water overdose without taking recent water consumption into careful consideration may have serious consequences. Immediately ceasing excessive water consumption may be enough to stop water overdose if caught soon enough, and diuretics may help in later stages. Before taking any but the most basic steps to counteract water overdose, professional medical assistance should be sought, especially given the potential for misdiagnosis by and amateur.