Can too much water kill you?

Drinking too much water causes 'water intoxication'.

The body fluids become diluted due to the excess of water compared to the normal solutes the fluids carry. This causes a large concentration gradient to develop - the fluids inside the body cells are more concentrated than the fluid surrounding them. This will cause water to enter the cells by osmosis, making them swell.

It is these swollen cells that endanger the organism, and can even be fatal. It may put pressure on, and block the flow of blood vessels to the brain and nervous system.