It’s rare, but you can drink too much water. When you do, your kidneys can’t get rid of the extra fluid fast enough. This drops the sodium in your blood to unhealthy levels -- a condition called hyponatremia. It can cause nausea, weakness and fatigue, headache, or, in severe cases, seizures, coma, and death. You’re at higher risk if you’re an athlete who exercises for long stretches of time, like a marathon runner.