it is theroticaly possible to drink to much water. there is a condition known as "water intoxication." It is usually associated with long distance events like running and cycling. And it’s not an unusual problem. For example, water intoxication was reported in 18% of marathon runners and in 29% of the finishers in a Hawaiian Ironman Triathlon in studies published recently in the Annals of Internal Medicine and in Medicine & Science in Sports & Exercise respectively.

What happens is that as the athlete consumes large amounts of water over the course of the event, blood plasma (the liquid part of blood) increases. As this takes place, the salt content of the blood is diluted. At the same time, the athlete is losing salt by sweating. Consequently, the amount of salt available to the body tissues decreases over time to a point where the loss interferes with brain, heart, and muscle function.

The official name for this condition is hyponatremia. The symptoms generally mirror those of dehydration (apathy, confusion, nausea, and fatigue), although some individuals show no symptoms at all. If untreated, hyponatremia can lead to coma and even death.