Most of the people I know don't drink enough water! So who gets intoxicated by drinking too much water? One group that is at risk are runners (especially marathoners). Hyponatremia is basically a low concentration of sodium in the blood. Sodium is required to draw and distribute water through membranes in the body. When you sweat, you lose water and salt. If the sodium levels in your blood get too low (hyponatremia), you will no longer be able to move water across your body's membranes and you will become dehydrated - even if you are drinking enough water.
Some of the signs and symptoms of hyponatremia include bloating, upset stomach, nausea, headaches, cramps, disorientation, slurred speech and confusion. Untreated, hyponatremia and dehydration can lead to collapse, convulsions, and sometimes even death.
Everyone is different, but generally you should try to ingest 1 gram of sodium per hour during a long event. You should also increase your sodium intake in the days leading up to the race. Ingest about 10-25 grams of salt per day before the race. You can't drink enough sports drink per hour during a long race (it would take about 2 liters of drink to get 1 gram of sodium). Salt tablets and salty foods (such as crackers or pretzels) are an option. Salt tablets and water are a good combination during a race.
It is also possible to become hyponatremic without sweating out all your salt. Over-hydration in a cooler climate can cause low sodium concentrations in the blood.
When preparing for a race, practice salt replacement while training, increase salt intake in the days leading up to the race, drink an amount of fluid which is appropriate to the race climate, and ingest salt during the race if the day is hot.