Water intoxication causes symptoms that range from nausea and headache in mild cases to seizures, coma, and death when severe. The underlying problem is dangerously low sodium in the blood: when you drink more water than your kidneys can process, your blood becomes diluted, and water shifts into your brain and other tissues by osmosis. Your kidneys can handle roughly 800 to 1,000 milliliters per hour under normal conditions, so symptoms typically appear when intake far exceeds that rate over a short period.
Early Warning Signs
The first symptoms of water intoxication are easy to mistake for something harmless. Nausea, a bloated stomach, and headache are the earliest signals that you’ve taken in too much water. Drowsiness and a general sense of feeling “off” often follow. These symptoms can look like a stomach bug, mild food poisoning, or simple fatigue, which is part of what makes water intoxication dangerous: people don’t always recognize what’s happening and may keep drinking.
One useful clue is your urine. Consistently colorless or completely clear pee suggests you’re overhydrated. Pale yellow is the normal target. If you notice clear urine alongside nausea or bloating, that combination should prompt you to stop drinking fluids and let your body catch up.
Muscle Symptoms
As sodium levels drop further, your muscles start to struggle. Weakness, cramps, and generalized muscle pain are common. These happen because sodium is essential for nerve signals that control muscle contraction. When blood sodium falls below its normal range (roughly 135 to 145 milliequivalents per liter), those signals misfire. Muscle cramps from water intoxication can feel identical to cramps from dehydration or electrolyte loss after exercise, which adds to the confusion.
Swelling in the Hands, Feet, and Face
Excess water doesn’t just stay in your bloodstream. It seeps into surrounding tissues, causing visible puffiness. Swelling in the hands, feet, belly, and sometimes the face is a hallmark of overhydration. If you notice your rings feel tight or your ankles look puffy after heavy fluid intake, that’s a sign your body is holding far more water than it needs. Weight gain during or after an endurance event (rather than the expected 1.5 to 2.5 kilogram loss) is another reliable indicator of fluid overload.
What Happens Inside the Brain
The most dangerous consequence of water intoxication is brain swelling. When the blood becomes too dilute, water moves into brain cells through osmosis, causing them to expand. The skull is rigid, so even modest swelling creates intense pressure. An increase in brain volume of just 8% to 10% can be fatal.
How quickly sodium drops matters more than the absolute number. A gradual decline over days may cause only mild symptoms, while a rapid drop over hours can trigger life-threatening complications even at sodium levels that might otherwise seem only moderately low.
Severe Neurological Symptoms
Once brain swelling progresses, the symptoms become unmistakable. Confusion, irritability, and slurred speech give way to more alarming signs: seizures, loss of consciousness, and coma. In the most extreme cases, the swelling pushes the brainstem downward (a process called herniation), which can cause abnormal posturing, dilated pupils, and sudden death. Difficulty breathing or frothy sputum can also appear, signaling fluid buildup in the lungs.
These severe symptoms represent a medical emergency. The progression from confusion to seizure can happen quickly, sometimes within hours of the onset of milder symptoms.
Who Is Most at Risk
Endurance athletes are the most commonly affected group. Marathons, triathlons, ultramarathons, and long-distance hikes create conditions where people drink aggressively over many hours, sometimes following outdated advice to consume as much fluid as possible. As that guidance became widespread, the incidence of exercise-related low sodium climbed noticeably. But the risk isn’t limited to elite events. Cases have been documented in football players, military personnel on desert operations, yoga practitioners, weightlifters, tennis players, and even musical theater performers during long rehearsals.
People with certain psychiatric conditions that drive compulsive water drinking are also at elevated risk. In these cases, the symptoms include the same progression of headache, nausea, lethargy, confusion, and seizures, but the cause may not be immediately obvious to those around them.
Infants Are Especially Vulnerable
Babies under six months face a disproportionate risk. Their kidneys are immature and far less efficient at excreting excess water. The most common cause is overly diluted formula or supplemental water given between feedings. Juices, soda, and tea have also been implicated. In infants, the telltale symptoms are irritability or unusual sleepiness, low body temperature, puffiness, and seizures. A total body water increase of just 7% to 8% can trigger symptoms in a baby.
Infants who are vomiting or have diarrhea are at particular risk if they’re given fluids that lack sufficient sodium, like plain water, instead of appropriate rehydration solutions. But water intoxication can also occur in otherwise healthy infants who simply receive too much plain water alongside their regular feedings.
How It Differs From Dehydration
Water intoxication and dehydration share several symptoms, including headache, dizziness, nausea, and confusion. This overlap is especially problematic for athletes, because the wrong response (drinking more water when you’re already overhydrated) makes things worse. A few features help distinguish them:
- Heart rate and blood pressure: Dehydration typically causes a fast heart rate and low blood pressure. In water intoxication, vital signs usually remain relatively normal.
- Physical appearance: Dehydration dries out the mouth, causes sunken eyes, and makes skin lose its elasticity. Overhydration causes puffiness and visible swelling.
- Body weight: If you’ve gained weight or haven’t lost weight during prolonged exercise, fluid overload is the more likely explanation. Losing more than 2% of body weight points toward dehydration.
When in doubt during an endurance event, the safest approach is to stop drinking until the situation is clearer rather than defaulting to more fluids.