Drinking too much water is called water intoxication, and the dangerous condition it triggers is known as hyponatremia. Hyponatremia means your blood sodium has dropped below the normal range of 135 to 145 milliequivalents per liter. While it sounds counterintuitive that something as harmless as water could be dangerous, drinking large amounts in a short period can overwhelm your body’s ability to maintain its chemical balance.
Why Too Much Water Is Dangerous
Your body relies on a careful balance of water and sodium to keep cells functioning properly. When you drink far more water than your kidneys can process, the excess dilutes the sodium in your bloodstream. This creates an imbalance where the fluid outside your cells is less concentrated than the fluid inside them.
When that happens, water follows a basic law of physics: it moves toward the higher concentration of sodium, flowing into your cells and causing them to swell. This is a problem everywhere in your body, but it’s especially dangerous in the brain. The skull is rigid, so there’s no room for the brain to expand. An increase in brain volume of just 8% to 10% can be fatal, according to research published in the Cleveland Clinic Journal of Medicine.
Symptoms From Mild to Severe
Water intoxication doesn’t go from fine to fatal without warning. The early signs are easy to dismiss: nausea, vomiting, bloating, and a headache. These can feel a lot like a stomach bug or simple overexertion, which is part of what makes the condition tricky to catch.
As sodium levels continue to drop, symptoms get more serious. You might notice drowsiness, muscle weakness, cramps, and swelling in your hands, feet, or belly. Mental changes like confusion, irritability, and dizziness are red flags that the condition is progressing. Without treatment, severe water intoxication can lead to seizures, delirium, coma, and death.
Who Is Most at Risk
Endurance athletes are one of the most commonly affected groups. Before 1981, athletes were actually told to avoid drinking during exercise, which sometimes led to dehydration. The guidance then swung in the opposite direction, encouraging athletes to drink as much fluid as possible. That well-meaning advice coincided with a rise in hyponatremia cases among marathon runners and ultramarathon participants, particularly in the United States. The condition was first formally described in endurance athletes competing in events longer than seven hours.
The core problem for athletes is straightforward: they lose sodium through sweat and then replace it with plain water, diluting what’s left. Smaller-bodied runners who drink at the same rate as larger competitors are at particular risk because the same volume of water has a bigger diluting effect.
Infants are another vulnerable group. Babies under 6 months old should not drink water at all. Their kidneys are immature, and even small amounts of water can dilute the sodium in their tiny bloodstreams enough to cause seizures, coma, or permanent brain damage. This also applies to formula that has been watered down. Breast milk and formula already contain all the hydration an infant needs.
Certain medications also raise your risk by interfering with how your kidneys handle water. Your body produces a hormone that tells the kidneys to hold onto water. Some common drug classes can cause the body to release too much of this hormone, making it harder to get rid of excess fluid. These include certain antidepressants (especially SSRIs), opioid painkillers, anti-seizure medications, antipsychotics, and some anti-inflammatory drugs like ibuprofen. If you take any of these, your threshold for “too much water” may be lower than average.
How Much Water Is Too Much
There’s no single number that applies to everyone, but a healthy adult’s kidneys can excrete roughly 0.8 to 1 liter of water per hour at maximum capacity. Drinking significantly more than that, especially over a short window, is where trouble starts. Most documented cases of water intoxication involve people drinking several liters within just a few hours.
For general daily intake, research suggests most healthy adults get enough fluid from about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day, and that includes water from food. Fruits, vegetables, soups, and other beverages all count toward that total. The old “eight glasses a day” rule is a reasonable baseline for most people, but the idea that more is always better is where the danger lies.
What Happens if You Need Treatment
Mild cases often resolve on their own once you stop drinking and let your kidneys catch up. Restricting fluid intake is the simplest first step, and for many people with modestly low sodium, that’s all it takes.
Severe cases with neurological symptoms like seizures or loss of consciousness are a medical emergency. In the hospital, the goal is to carefully raise blood sodium levels back toward the normal range. This is done with a concentrated salt solution given intravenously. The correction has to be gradual because raising sodium too quickly carries its own risks, including a rare but serious form of brain damage.
The speed of onset matters. Hyponatremia that develops over hours (acute) is more dangerous than a slow decline over days because the brain hasn’t had time to adapt. Acute cases, like those from a water-drinking contest or an intense athletic event, require the most urgent intervention.
Practical Ways to Stay Safe
Thirst is a surprisingly reliable guide for most healthy adults. Drinking when you’re thirsty and stopping when you’re not is a simple strategy that works well in everyday life. During exercise, matching your fluid intake roughly to your sweat losses is safer than forcing yourself to drink on a schedule. Sports drinks containing sodium can help during prolonged activity lasting more than an hour.
Checking your urine color is another easy gauge. Pale yellow means you’re well hydrated. Clear and colorless urine, especially if you’re urinating very frequently, can be a sign you’re overdoing it. Dark yellow means you need more fluid. The goal is the middle ground, not the extremes.