Can Drinking Water Dehydrate You? Hydration Facts

No — water itself can't dehydrate you. But drinking very large amounts of plain water in a short time, especially during long endurance exercise, can dilute the sodium in your blood. That condition, called hyponatremia, is rare but can be dangerous.

How hydration works

Your body keeps the balance of water and sodium within a narrow range. Thirst and the kidneys do most of the work: if you drink more than you need, the kidneys excrete the excess — up to about 0.8–1 liter per hour. Problems arise only when intake outpaces that for hours.

When too much water becomes a problem

• Marathons, triathlons and long hikes where people drink large volumes of plain water while losing sodium in sweat
• Drinking contests or forced fluid intake
• Certain medications (thiazide diuretics, some antidepressants) and conditions (heart, kidney or liver disease) that impair water excretion

Symptoms of hyponatremia: nausea, headache, confusion, swelling of hands and feet, and in severe cases seizures. It needs urgent medical care.

How much should you drink?

Total water needs (from drinks and food) are roughly 2.7 liters a day for women and 3.7 liters for men — about 20% of it comes from food. Needs rise with heat, exercise, fever and breastfeeding. For most healthy people, drinking to thirst and having pale yellow urine are good guides. During long exercise, drink to thirst and include sodium (sports drinks or salty snacks) when sweating for more than an hour or two.

Signs of dehydration

• Thirst, dry mouth, dark yellow urine, urinating less
• Tiredness, headache, dizziness when standing up
• In older adults: confusion — the sense of thirst weakens with age

Hydration and blood pressure

Dehydration lowers blood volume and can cause low blood pressure and dizziness on standing; it can also make the heart beat faster. Drinking enough is one of the first steps for people with low blood pressure (see low blood pressure tips). People with heart failure or advanced kidney disease may have a fluid limit — follow your doctor's advice.

Do coffee and tea count?

Yes. Moderate amounts of coffee and tea contribute to daily fluid intake; their mild diuretic effect does not cause net dehydration in regular drinkers. Alcohol, on the other hand, increases urine output.