In 2002 35-year-old Hillary Bellamy collapsed at the Marine Corps Marathon in Washington, D.C. The next day she died at a nearby hospital. The cause: hyponatremia, a disorder caused by excessive fluid consumption that as a result dilutes the blood’s salt content to low concentration levels. The rare condition has recently crept into the mainstream parlance of runners and endurance athletes. Dehydration is now not the only thing to fear. Over-hydration can be just as dangerous.

These days we live in a culture that is H2O obsessed. Most people never leave home without their 20-ounce bottle of Dasani. And while eight glasses a day may be a key to better health, it’s becoming apparent that when it comes to water, there can be too much of a good thing.

The condition usually occurs in athletes taking on distance events lasting longer than four hours. Sodium is an electrolyte that helps with water distribution. The more people sweat, the more salt they lose.

And it’s not just limited to the active. In mid-January, a California woman died from “water intoxication” after participating in a radio station challenge to drink as much water as possible without going to the bathroom. She reportedly drank almost two gallons in an effort to win her kids a Nintendo.

Hyponatremia cases have increased as more people attempt to run longer distances. According to Running USA, in 1980 an estimated 120,000 people finished a marathon at a median time of 3:32:17 for men and 4:03:39 for women. In 2005 432,000 people ran races of 26.2 miles averaging 4:20:29 (male) and 4:51:19 (female). More people with average athletic abilities are attempting longer feats. They are out on the course, sweating for nearly five hours. Bellamy was attempting her first marathon.

Coaches notice that a common problem with new runners is drinking too much. Randy Ashley, a coach at ZAP Fitness in Blowing Rock, N.C., helped a woman improve her marathon time from 5:05 to 4:17 by significantly reducing her water stops.

“Hydration strategy is vital to getting people across the line healthy,” says Ashley. “You don’t need to stop every two miles."

Symptoms of on-setting hyponatremia can vary and include mild disorientation, lightheadedness, bloating, and nausea, as well as seizures and coma in severe cases.

Another D.C. runner, Michele Burr, survived a near-fatal bout with hyponatremia after running the Vermont 100-Miler. Proof that this can even happen to the seasoned athlete, this was Burr’s fifth 100-miler, but at the end of the race she did not recognize her husband waiting for her at the finish line.

“I walked the last five to 10 miles, which is very unusual for me, and people said I didn’t know who they were and it appeared as though I didn’t even know I was in a race,” Burr said in her post-race report.

Soon after finishing the race, Burr started vomiting, then after having a seizure, she slipped into a coma for three days. She spent five days in the hospital before starting a slow recovery.

While hyponatremia can have serious consequences, athletes can easily be proactive in prevention. During exercise lasting longer than an hour, it’s a good idea to put down the water in exchange for a sports drink that replaces essential electrolytes, including sodium and potassium.

“It’s probably not going to be a problem for the average weekend warrior that’s running for 45 minutes,” says Dr. Michael Saunders, associate professor of exercise physiology at James Madison University. “People need to gauge what they are doing carefully. If you know you’re going to be engaging in an activity where you’ll be losing a lot of fluids and electrolytes, you want to try and replace as much as you’re losing.”

But there is no magic bullet or standard formula for avoiding the condition. Some people sweat more than others, and some people lose more salt than others when they sweat. Famous Olympic marathoner Alberto Salazar struggled in hot temperatures because he sweat 3.5 liters per hour, much more than the average person.

The American College of Sports Medicine says athletes taking on longer events should try to drink to match fluid loss. Athletes should know their hourly sweat rate, which is calculated by adding weight lost during exercise per hour plus any fluid consumed during exercise per hour, and aim to replace that total amount. Energy gels, snacks containing sodium, and salt tablets are also good ways to help offset the loss of salt in sweat.

“Common sense needs to prevail over competitive drive,” says Saunders. “This is not an everyday occurrence, but when it does happen it can be quite serious, so athletes need to pay attention.”