Normally, sodium is able to balance the fluids in and around your cells.

But drinking too much water causes the liquid to move from your blood into your cells, which makes them swell. In some places in your body, this isn't a problem. But in your brain, there's little room to accommodate the larger cells, which can result in brain swelling. Brain swelling could be followed by dangerous conditions such as seizures or a coma.

How much is too much?

WebMD does not offer an exact answer, but the site’s experts recommend drinking until you don't feel thirsty and then stopping. Also check your urine when you go to the bathroom – it should be light-colored, but not clear. And it can help to have sports drinks (in a moderate amount) if you're exercising intensely, since these contain sodium.

How common is it?

Fortunately, your kidneys are usually able to excrete the excess water you take in, so hyponatremia is fairly rare in otherwise healthy people. Endurance athletes who drink a lot of water can be at higher risk.

A study in the New England Journal of Medicine found that nearly one-sixth of marathon runners develop hyponatremia to some degree, according to Scientific American. The reasoning behind those statistics is that in times of physical stress, your body can produce more vasopressin, a hormone that tells the kidney to conserve water. So the combination of taking in a lot of water at a time when your kidneys are conserving it can cause hyponatremia.

The publication also cites some unfortunate instances where hyponatremia led to death. For example, a woman died after participating in a radio station's water-drinking contest and a man who died as a result of drinking excessive amounts of water as part of a fraternity hazing.

What to look out for

The following are some common symptoms of this condition also known as water intoxication:

Confusion

Headache

Fatigue

Convulsion

Muscle spasms, cramps or weakness

Nausea or vomiting

Irritability

If you suspect hyponatremia, get medical help immediately.