Water poisoning, also called water intoxication, happens when you drink so much water that your blood sodium levels drop to dangerous lows. The excess water dilutes sodium in your bloodstream, causing cells throughout your body to swell. When brain cells swell, the consequences can be fatal. Normal blood sodium sits between 135 and 145 milliequivalents per liter; symptoms typically appear once levels fall below 120, and severe, life-threatening complications set in around 90 to 105.
How Excess Water Overwhelms Your Body
Your kidneys are remarkably efficient, but they have a ceiling. Healthy kidneys can excrete roughly 800 to 900 milliliters of fluid per hour. If you’re drinking faster than that, water accumulates in your bloodstream and begins diluting sodium and other electrolytes. The typical daily fluid recommendation for adults is about 11.5 to 15.5 cups (2.7 to 3.7 liters) from all sources combined, including food. Consuming several liters in a short window, especially within a few hours, is what pushes the body past its limits.
Sodium controls how water moves in and out of cells. When blood sodium drops, the fluid balance tips: water floods into cells because the concentration of dissolved particles is higher inside them than in the now-diluted blood. Cells throughout the body swell, but the brain is uniquely vulnerable. Unlike muscle or skin, the brain is enclosed in a rigid skull with no room to expand. Swelling brain tissue increases pressure inside the skull, and that pressure is what drives the most dangerous symptoms of water poisoning.
Symptoms From Early to Severe
Early water intoxication is easy to miss because the initial symptoms look like many other conditions. Nausea, vomiting, and a general feeling of confusion or disorientation come first. Some people develop headaches, lethargy, or restlessness. In clinical case reports, early-stage water intoxication has been mistaken for psychosis because patients sometimes exhibit bizarre behavior, delusions, or hallucinations before anyone suspects a sodium problem.
As sodium continues to fall, the progression is predictable and increasingly serious: confusion deepens into drowsiness, then seizures, then coma. At very low sodium concentrations, the brain swells enough to push against the base of the skull, a process called herniation. Heart rate may slow and blood pressure may spike as the body tries to compensate for rising pressure inside the skull. Without treatment at this stage, death can follow.
The speed of onset matters enormously. A gradual sodium decline over days gives the brain some time to adapt by shedding internal solutes. A rapid drop over hours is far more dangerous because the brain has no time to adjust.
Who Is Most at Risk
Endurance athletes are one of the best-studied risk groups. Research on marathon runners found that slower runners, particularly those finishing in over four hours, face a higher risk than elite competitors. The reason is straightforward: they’re on the course longer, passing more drink stations, yet their lower running intensity means they sweat less and need less fluid replacement. Many of these runners gain weight during the race from excess fluid intake, which is a red flag for developing dangerously low sodium. In ultra-endurance events like Ironman triathlons and ultramarathons, the situation is compounded by prolonged sodium loss in sweat, which means drinking large volumes of plain water without replacing sodium creates even more severe dilution.
People with certain psychiatric conditions, particularly psychogenic polydipsia (compulsive water drinking), are another well-documented risk group. Military recruits in hot climates have also been affected, often after being instructed to drink aggressively to prevent dehydration during training.
Infants Under Six Months
Babies are especially vulnerable to water poisoning for two reasons: their kidneys are still immature and less capable of excreting excess water, and their small body size means even modest amounts of extra fluid can shift sodium levels dramatically. A total body water increase of just 7 to 8 percent can trigger symptoms in an infant, including irritability, abnormal sleepiness, low body temperature, facial swelling, and seizures.
The most common cause in infants is diluted formula or supplemental water given between feedings. Breast milk and properly mixed formula already contain all the water a baby needs. The CDC has noted that tap water, bottled water, juice, soda, and tea have all been implicated in infant water intoxication cases. Babies under six months generally should not be given supplemental water at all. If a young child has vomiting or diarrhea, oral rehydration solutions that contain sodium are the appropriate replacement, not plain water.
How Water Poisoning Is Treated
Treatment depends on how severe the sodium drop is and how quickly it happened. Mild cases may only require fluid restriction, letting the kidneys catch up and restore normal sodium levels on their own.
Severe cases with seizures, coma, or signs of brain herniation are medical emergencies. In the hospital, doctors use concentrated salt solutions given intravenously to raise blood sodium in a controlled way. The goal in the first hour is to raise sodium by about 4 to 6 milliequivalents per liter, enough to reduce brain swelling and stop seizures. After that initial correction, the pace slows considerably. Correcting sodium too fast carries its own serious risk: a condition called osmotic demyelination syndrome, which damages the protective coating on nerve cells in the brain and can cause permanent neurological injury. Doctors carefully monitor blood sodium levels every few hours and adjust treatment to avoid overcorrection.
How Much Water Is Too Much
There’s no single number that applies to everyone because body size, kidney function, sweat rate, and sodium intake all play a role. But the kidney’s excretion ceiling of roughly 800 to 900 milliliters per hour provides a useful benchmark. Drinking consistently above that rate, especially if you’re consuming plain water without food or electrolytes, puts you in the danger zone.
For endurance athletes, the practical advice that has emerged from research is to drink based on thirst rather than following a fixed schedule. Weight gain during exercise is a warning sign that you’re drinking more than you’re losing. For ultra-endurance events, replacing sodium alongside fluids is essential to prevent dilution even when total volume seems reasonable.
For everyday situations, water poisoning is rare. Most healthy adults with functioning kidneys and a normal diet would have to drink an extreme amount in a short period to develop symptoms. The cases that make headlines typically involve water-drinking contests, hazing rituals, or people who deliberately force fluids far beyond thirst. The risk rises when any of those factors that slow kidney excretion or reduce sodium, such as certain medications, intense prolonged exercise, or very low food intake, are also present.