It used to be that you weren’t supposed to drink when you ran. Then, you were supposed to drink at every opportunity. Now, experts say you should drink only when thirsty. What gives?

Also known as water intoxication, hyponatremia became well known after a woman competing in a “Hold your Wee for a Wii” contest run by a radio station died after consuming way too much water in a very short period of time. But while it might seem like over-hydration is something only Darwin Award competitors need worry about, it’s a real threat for a certain subset of outdoor enthusiast: the recreational athlete, says Dr. Douglas Casa, chief operating officer of the Korey Stringer Institute at the University of Connecticut.

“Elite athletes aren’t the ones getting hyponatremia,” he says. “An elite marathoner doesn’t have the time to overdrink. You see it in the recreational person who stops at every aid station.”

WHAT HAPPENS: You’re drinking too much water over too long a period of time. And for some reason, your brain thinks you’re dehydrated and stops urine production. You keep sweating, but it’s not enough. Soon, your tissues become bloated, then the brain swells and pushes against the skull, cutting off blood supply. If it affects the parts of the brain that control breathing, you’re a goner.

SIGNS AND SYMPTOMS: Somewhat similar to heat stroke. Headache and nausea, vomiting, altered mental status, bloating.

PREVENTION: If you're not thirsty, don't hydrate. “For recreational athletes, drink to your thirst,” says Casa. “Not beyond it. And for elite athletes, develop a hydration plan.”

TREATMENT: Drink less water, eat salty foods, consume oral hypertonic saline.

THE MYTHS: It’s impossible to over-drink because you'll pee it out. Water intoxication occurs only during exercise.

THE REALITY: You can over-drink. Hyponatremia can occur hours after exercise if lost sodium is not made up for.

THE BOTTOM LINE: If you're not thirsty, don't drink.