Overhydration & Water Intoxication: Symptoms, Risks, Prevention

While most conversation about hydration focuses on drinking more water, the opposite extreme — drinking too much water too quickly — is a real and dangerous condition. Water intoxication, also known as hyponatremia, occurs when the body's sodium levels drop dangerously low due to excessive water intake. This guide explains what hyponatremia is, who is at risk, how to recognize it, and how to prevent it.

What Is Hyponatremia?

Hyponatremia is defined as a blood sodium concentration below 135 milliequivalents per liter (mEq/L). Normal sodium levels range from 135 to 145 mEq/L. Sodium is the primary electrolyte in blood and the extracellular fluid, and it plays a critical role in maintaining fluid balance, nerve function, and muscle contraction. When sodium levels drop too low, water moves from the blood into cells to balance the concentration gradient. Most cells can accommodate this swelling, but the brain cannot — it is enclosed in the rigid skull, and brain swelling (cerebral edema) can be life-threatening.

There are two main types of hyponatremia relevant to otherwise healthy people. Exercise-associated hyponatremia (EAH) occurs in endurance athletes who drink large amounts of plain water during prolonged events, diluting their blood sodium. The condition was first identified in the 1980s among marathon runners and has since been documented in ultramarathoners, triathletes, hikers, military trainees, and even participants in fraternity hazing rituals involving forced water consumption.

The second type is psychogenic polydipsia, a condition sometimes seen in people with psychiatric disorders (particularly schizophrenia) where compulsive water drinking leads to hyponatremia. Certain medications, including some antidepressants and the recreational drug MDMA (ecstasy), can also cause or contribute to hyponatremia through effects on antidiuretic hormone.

Symptoms of Water Intoxication

Early symptoms of hyponatremia are often subtle and can be mistaken for ordinary fatigue or dehydration, which makes the condition particularly dangerous. Early signs include headache, nausea, vomiting, confusion, fatigue, muscle weakness or cramps, and irritability. As sodium levels continue to fall, more severe symptoms develop: seizures, hallucinations, loss of consciousness, coma, and in extreme cases, death from brain herniation.

A critical mistake people make is assuming that the symptoms of hyponatremia mean they need to drink more water. If an endurance athlete feels nauseous and confused after a long race, the instinct may be to drink more — but if the cause is hyponatremia, drinking more water makes it worse. This is why race medical teams now weigh athletes and check sodium levels rather than simply handing out more water.

If you or someone near you shows signs of severe hyponatremia — particularly seizures, confusion, or loss of consciousness — seek emergency medical attention immediately. Treatment involves carefully administered hypertonic saline in a hospital setting to gradually raise blood sodium levels. Rapid correction of sodium can itself cause brain damage (a condition called osmotic demyelination syndrome), so treatment must be medically supervised.

Who Is at Risk?

The populations at highest risk for water intoxication are endurance athletes, military trainees, hikers, and people using certain recreational drugs. Marathon and ultramarathon runners are at particular risk because the combination of prolonged sweating (which loses both water and sodium) and aggressive plain water replacement can dilute blood sodium over many hours. The risk is highest in events lasting more than 4 hours, in slower runners (who have more time to drink), in smaller runners (who have less body water to dilute), and in hot conditions where sweat losses are high.

Military trainees have died from water intoxication during intense training exercises where they were encouraged to drink large amounts of water rapidly. As a result, military training protocols now include specific guidelines on maximum water intake rates. Hikers, particularly those on long trails like the Appalachian Trail or Pacific Crest Trail, are also at risk if they drink large amounts of plain water at water sources without consuming electrolytes.

The recreational drug MDMA (ecstasy) is a known cause of hyponatremia, both because the drug can cause excessive water drinking (users feel extremely thirsty) and because it triggers inappropriate release of antidiuretic hormone, which prevents the kidneys from excreting excess water. Several deaths have been attributed to MDMA-associated hyponatremia at music festivals and dance parties.

How Much Water Is Too Much?

The kidneys of a healthy adult can excrete approximately 0.8 to 1.0 liters of water per hour. Drinking more than this rate overwhelms the kidneys' ability to eliminate the excess, and blood sodium begins to dilute. The general guideline is to drink no more than 1 liter per hour, with most of that spread out rather than gulped. For endurance athletes, the recommendation is to drink to thirst rather than following a rigid drinking schedule, and to include electrolytes (particularly sodium) in any fluid consumed during events lasting more than 2 hours.

A widely cited 2005 study in the New England Journal of Medicine examined Boston Marathon runners and found that 13 percent had hyponatremia at the finish line. The strongest predictors were weight gain during the race (indicating overdrinking), racing time over 4 hours, and low body mass index. The study led to significant changes in how endurance events are organized, with race directors now advising runners to drink to thirst rather than drinking at every aid station.

For most people going about daily life, water intoxication is not a realistic concern. The risk comes from drinking large volumes of water in a short period, particularly during prolonged exercise or in response to drugs that affect fluid regulation. Drinking the standard recommended amount of water spread across the day is safe for healthy adults.

Prevention Strategies

For endurance athletes, the most important prevention strategy is to drink to thirst rather than forcing fluid intake. The International Marathon Medical Directors Association recommends that runners drink no more than 400 to 800 ml per hour, with the lower end for smaller and slower runners. For events lasting more than 2 hours, choose sports drinks containing sodium over plain water, or consume salty snacks along with water. Avoid taking nonsteroidal anti-inflammatory drugs (NSAIDs like ibuprofen) during endurance events, as they can impair kidney function and increase hyponatremia risk.

For hikers and backpackers on long trips, carry electrolyte tablets or powder and add them to water, particularly in hot conditions where sweat losses are high. Eat salty snacks like pretzels, trail mix with salted nuts, or jerky. Pay attention to thirst and do not force large amounts of water at every water source.

For the general public, the message is simpler: drink when thirsty, spread intake across the day, and do not drink more than about 1 liter per hour. If you have a kidney condition, heart failure, or are taking medications that affect fluid balance, follow your healthcare provider's specific guidance on fluid intake. Calculate your baseline daily target with our Daily Water Intake Calculator and aim to hit that target gradually rather than all at once. For more on the broader topic of fluid balance, see our guide to electrolytes explained.