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You've heard it a million times. When it's hot outside or you're exercising, drink lots of water. It's how your body stays hydrated.

But can there be too much of a good thing? In rare cases, drinking an extreme amount in a short time can be dangerous. It can cause the level of salt, or sodium, in your blood to drop too low. That's a condition called hyponatremia.It's very serious, and can be fatal. You may hear it called water intoxication.

How much would you have to drink? An enormous amount. Gallons and gallons of water.

"These are very isolated cases, and this is extremely rare," says Sharon Bergquist, MD. She's an assistant professor of medicine at Emory University School of Medicine in Atlanta. "More people by far and away are dehydrated, [rather] than having a problem with over-hydration."

What Is Water Intoxication?

If you drink a bottle of water here and there when you exercise or when you're hot, you’ll be fine. Where you run into problems is drinking way too much too fast.

"Young, healthy people don’t normally [get hyponatremia] unless they drink liters and liters of water at once, because your kidneys can only [expel] about half a liter at most an hour," says Chris McStay, MD. He's an emergency medicine doctor at the University of Colorado School of Medicine. "You're drinking more than your kidneys can pee out."

Causes

The issue boils down to sodium levels. One of sodium’s jobs is to balance the fluids in and around your cells. Drinking too much water causes an imbalance, and the liquid moves from your blood to inside your cells, making them swell. Swelling inside the brain is serious and requires immediate treatment.

Sometimes babies can have issues. Their bodies are so tiny that they can't handle lots of water. That's why doctors say infants should drink only milk or formula.

Then there are the cases like hazing rituals and publicized contests where people drink large amounts on purpose.

Symptoms and Treatment

The warning signs of hyponatremia look a lot like the symptoms of heatstroke and exhaustion. You might be hot, have a headache, and just feel crummy. Other early symptoms can include diarrhea, nausea, and vomiting.

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