Yes, it can be dangerous, but it depends on the circumstances. Most people, with normal kidneys, can pee out the extra water. But, if you overwhelm the system, you can develop hyponatremia, or a low sodium concentration in the blood. This is a severe, life-threatening emergency, requiring intensive medical care. Most people who drink too much water have either psychogenic polydipsia (i.e. they have a mental disease and drink too much water) or have taken ecstasy. I’ve seen several cases of young kids who almost died from E.