Why can even water become poisonous when too much is consumed?

Water is essential for life, but it can become toxic when consumed in excessive amounts, a condition known as overhydration. This occurs when the intake of fluid exceeds the body's ability to excrete it, or when regulatory mechanisms—such as the secretion of vasopressin—are defective.1

The human body maintains a delicate fluid and electrolyte balance by keeping concentrations of minerals like sodium and potassium within narrow, vital ranges. When an individual consumes too much water, the concentration of these electrolytes in the extracellular fluid becomes dangerously diluted. Because the body’s cells and fluid compartments are closely integrated, this imbalance disrupts the osmotic pressure and the electrical potential of cell membranes.234

While the kidneys typically regulate water levels by adjusting urine output, they can be overwhelmed by rapid, excessive intake. This state, sometimes called water intoxication, can lead to serious physiological dysfunction because the body requires precise ionic concentrations to maintain proper heart rhythm, nervous system impulses, and overall cellular function.213