As dangerous as dehydration is, it is also possible to overdose on water. You may have heard this referred to as “ water intoxication ,” but the correct term for it is hyponatremia . It happens when the level of sodium in your blood gets too low. Most commonly, it’s caused by an underlying health issue or a medication that makes you sweat a lot, but it can also happen when you drink too much water in a short period of time, which dilutes the sodium in your blood.For people without underlying health issues, the risk of hyponatremia is highest during the times you’re sweating and drinking a lot of water, so it’s most often a problem among marathon runners or other athletes.Although it’s still considered rare, it might be more common than previously thought. A 2005 study of runners participating in the Boston Marathon found that 13% of runners had hyponatremia in provided blood samples at the end of the race, even though they didn’t have any symptoms (nausea is one common sign). Even though 13% is a relatively small number, this is still a big deal, because a rapid drop in sodium levels can lead to swelling in the brain, which can be life-threatening and requires immediate medical attention. Per the Mayo Clinic , reproductive-age women are most at risk for serious brain complications of hyponatremia, possibly because of the way female sex hormones affect sodium balance.Following the deaths of two high school football players from hyponatremia, the Clinical Journal of Sports Medicine published a report from a group of 17 exercise scientists urging athletes (and coaches) to listen to their bodies rather than following more strict hydration routines. "Our major goal was to re-educate the public on the hazards of drinking beyond thirst during exercise," one of the authors of a report said in a statement For regular gym-goers, this isn't a huge concern. But if you're training for a marathon or doing another activity that requires long periods of exercise, the best thing to do is drink only when you're thirsty, rather than worrying about replacing a certain amount of fluid every hour on the hour, as previously recommended. It's also a good idea to choose sports drinks that have electrolytes such as sodium in them, to help keep your levels up.