NEW YORK (Reuters Health) - Runners who compete in "ultra-endurance" races sometimes develop a potentially dangerous condition caused by drinking too much water, a new study shows.

The condition, known as hyponatremia, occurs when blood sodium levels become abnormally low. In recent years, it's been recognized as a growing problem among marathoners who drink large amounts of water during competition, which dilutes the blood's sodium content.

While hyponatremia often causes relatively mild symptoms like headache, cramps and nausea, severe overhydration can lead to disorientation, seizures, coma or even death.

Less experienced runners seem to be at greatest risk; because they take much longer to finish the race than elite athletes do, their water intake can be far greater.

The new study, published in the Clinical Journal of Sports Medicine, focused on runners in an ultra-endurance mountain race - a grueling 37-mile journey that took the athletes 5 to 10 hours to complete.

Of 123 runners with complete data, 4 percent showed signs of mild hyponatremia by the end of the race, according to the researchers, led by Dr. Tony Page of SportsMed Canterbury in Christchurch, New Zealand.

Another 7 percent gained weight - a sign of excess fluid retention - but maintained normal sodium levels. This suggests that there might be "underlying mechanisms for sodium regulation" that protect some runners from hyponatremia, Page told Reuters Health.

"This needs further research," he said.

In other findings, there was no evidence of a higher hyponatremia risk among runners who used non-steroidal anti-inflammatory drugs (NSAIDs) before the race. This is in contrast to an earlier study that linked NSAID use to hyponatremia among triathletes.

NSAIDs include over-the-counter painkillers like aspirin, naproxen (Aleve) and ibuprofen (Motrin, Advil). In theory, the drugs may promote hyponatremia because they tend to inhibit water loss.

The lack of an association between NSAIDs and hyponatremia in this study does not mean it's safe for endurance runners to down painkillers before a race, Page cautioned. The finding, he noted, may simply stem from the fact that the study sample was relatively small.

"We should continue to urge caution in the use of NSAIDs in endurance runners, especially in older athletes," Page advised. It's a particular concern with older adults because kidney function naturally declines with age, he explained.

The International Marathon Medical Directors Association recently issued a new position statement intended to help prevent hyponatremia. It advises runners to trust their thirst to tell them when they need water, rather than trying to drink as much as possible to replace the fluid they lose through sweating.