While drinking fluids during the race is important for all runners, in some cases too much drinking can also be dangerous. Drinking more than one loses during a race can decrease the concentration of sodium in the blood (a condition called hyponatremia), which may result in vomiting, seizures, coma and even death. Eating salt packets during a race mitigates this problem. The International Marathon Medical Directors Association issued a warning in 2001 that urged runners only to drink when they are thirsty, rather than "drinking ahead of their thirst."

Women are more prone to hyponatremia than men. A study in the "New England Journal of Medicine" found that 13% of runners completing the 2002 Boston Marathon had hyponatremia.

A 4+ hour runner can drink about 4-6 fluid ounces (120-170 mL) every 20–30 minutes without fear of hyponatremia. Consuming sports drinks or salty snacks will also reduce the risk. A patient suffering hyponatremia can be given a small volume of a concentrated salt solution intravenously to raise sodium concentrations in blood. Some runners weigh themselves before running and write the results on their bibs. If anything goes wrong, first aid workers can use the weight information to tell if the patient had consumed too much water.

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