Over-hydration (hyponatremia) is one of the most common medical complications in long-distance training and racing. Many runners tend to be so focused on the performance impact of dehydration that they end up hydrating in excess. Novice and slower runners who mainly drink water are the most susceptible to this condition.
Some symptoms of over-hydration include: headache, confusion/disorientation, nausea/vomiting, and muscle weakness. If left untreated or misdiagnosed as dehydration, over-hydration can lead to seizure, brain swelling, fluid buildup in the lungs, coma, cardio-respiratory arrest, or death.
To prevent this, the maximum rate at which the intestines can absorb fluid is, on average, about 600 mL (20.3 oz.) per hour. If fluid is ingested above these rates, it will be retained and may cause a number of problems associated with over-hydration.
So, simply drink according to your thirst to stay safe and maximize your performance.