Here in the U.S. where we walk about with bottles of Evian holstered to our hips like wary gunfighters at a heavily-hydrated O.K. Corral, it may be tough to imagine drinking too much water. But it is possible, and it creates a condition known as water intoxication.

Don't laugh.

The condition causes headache, nausea, and vomiting. In more severe cases it may produce confusion, double vision, drowsiness, cramps, muscle weakness, and even seizures, brain damage, coma and death.

Quite simply, when you drink too much water, you can dilute the electrolytes in your blood – particularly sodium. And when sodium levels drop, fluids shift from the outside of your cells to the inside, causing them to swell. This condition is called hyponatremia.

When it's your brain cells that are doing the swelling, any number of the harmful symptoms listed above can kick in.

How can you possibly drink so much water your brain swells? It's been known to happen to soldiers, three of whom even died due to hyponatremia and cerebral edema after drinking 2.5 to 5.6 gallons of water in just a few hours.

Ironically, marathon runners – those athletes helping themselves to cups of water offered by well-meaning fans along their race routes – have also been known to quench a cup too far. At the 2002 Boston Marathon, 13 percent of participants had hyponatremia symptoms, with 0.06 percent showing critical hyponatremia. At another marathon, an over-hydrated competitor developed hydrocephalus and brain stem herniation, resulting in his death.

In addition to soldiers and athletes, water intoxication has been known to occur in schizophrenics. One study of 27 schizophrenics that had died young showed that five of them died due to water intoxication.

Water intoxication is not just about how much water you drink, but how quickly you have imbibed it. Your kidneys can only void about 27 to 33 ounces of liquid per hour. Out-pace that with your libations and you are courting hyponatremia. A case of water intoxication and prolonged hyponatremia occurred in a healthy, 22-year-old male prisoner after he drank 1.5 gallons of water in 3 hours.

So how much and how fast should you drink? Listen to your thirsts, and remember that your body is also extracting water from the solid foods you ingest. Pregnant women, of course, will need to drink for themselves and some extra for their unborn child.