Hyponatremia
Hyponatremia is most often a complication of other medical illnesses in which excess water collects in the body faster than can be excreted (for example in congestive heart failure, syndrome of inappropriate antidiuretic hormone (SIADH), or polydipsia). Some runners who drink a lot of water before running a marathon also suffer this illness. Drinking too much water  can impair the ability of marathon runners.