Drinking many gallons of water can cause hyponatremia, according to WebMD. The condition occurs when heightened levels of water in the body cause sodium levels in the blood to fall to insufficient levels.
Hyponatremia, or water intoxication, is a serious condition in which the level of salt in the blood drops below 135 milliequivalents per liter, notes Mayo Clinic. In extreme cases, it can be fatal. Although drinking water during activities such as exercise is not problematic, hyponatremia does become a concern when too much water is ingested too quickly and the kidneys cannot expel it fast enough, resulting in a sodium imbalance that causes the body’s cells to swell, explains WebMD.