Ok so they might not die. But it can cause something really bad called water intoxication.
So what is water intoxication?
Water intoxication is a condition that happens when the electrolytes in the body get diluted. Plain water does not have sodium and potassium in it. When you ingest large amounts of water your blood eventually gets diluted with fluid with no electrolytes in it.
Think of it this way- your blood is a pool with the sodium and potassium your body needs in it. Now dump water in the pool. You have more volume in the pool but you have less sodium and potassium because your pool now has a diluted solution in it.
Low amounts of sodium can cause seizures and coma.
Low amounts of potassium can cause cardiac abnormalities.
This can be fatal.
So how much is too much? It's hard to say. Babies less than 6 months do not need any free water at all. If you care concerned about dehydration then you can give them more formula or breastmilk. If they can't take formula then give them pedialyte.
This becomes an issue in an older child when they have vomiting and diarrhea and the parent is concerned about dehydration and gives them large amounts of water. In this case give them an electrolyte solution like pedialyte.
There have been cases of college students dying from water intoxication from drinking too much water in hazing incidents.. in an older child and adult it's less likely for this to happen but it's still possible.
Just use common sense.
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