Drinking too much water dilutes the sodium in your blood, and when levels drop low enough, your cells begin to swell. This condition, called hyponatremia, can range from mildly uncomfortable to life-threatening. A healthy blood sodium level sits between 135 and 145 millimoles per liter, and problems start once it falls below 135.
How Excess Water Affects Your Body
Your kidneys can only flush out a limited amount of water per hour. When you drink faster than your kidneys can keep up, the extra water enters your bloodstream and dilutes the sodium dissolved there. Sodium is the main mineral your body uses to regulate fluid balance between cells and the surrounding tissue. When sodium concentration drops, water moves into your cells through osmosis to try to equalize the balance.
Most cells can tolerate some swelling, but brain cells are trapped inside a rigid skull. As they absorb water and expand, pressure builds rapidly with no room to accommodate it. This is why the most dangerous symptoms of overhydration are neurological: confusion, seizures, and in extreme cases, coma or death.
Symptoms From Early to Severe
The earliest signs that you’ve had too much water are nausea, a bloated stomach, and a headache. These can appear well before sodium levels drop to dangerous territory, and they’re your body’s signal to stop drinking.
As sodium drops further, symptoms expand to include drowsiness, muscle weakness, cramps, and swelling in the hands, feet, and belly. You may also notice changes in how you think and feel: irritability, confusion, dizziness. At this moderate stage, sodium has typically fallen into the 125 to 129 range.
Below 125, the situation becomes a medical emergency. Severe water intoxication can cause seizures, delirium, coma, and death. These cases require immediate hospital treatment, usually involving concentrated saline solutions delivered intravenously to bring sodium back up in a controlled way. Recovery at this stage happens in an intensive care setting with frequent blood monitoring over hours or days.
How Much Water Is Too Much?
There’s no single cutoff, because your body’s ability to process water depends on kidney function, body size, activity level, and how quickly you’re drinking. General guidance suggests most healthy adults get adequate hydration from roughly 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day, including fluid from food. That’s a daily total, not a per-hour target.
The danger comes less from daily volume and more from rate. Chugging large amounts in a short window overwhelms the kidneys. Sipping steadily throughout the day rarely causes problems for someone with normal kidney function. If you’re drinking enough that your urine is consistently clear and you feel bloated or nauseous, you’ve likely overshot what your body needs.
Who Is Most at Risk
Endurance athletes are the most well-studied group for exercise-related overhydration. Marathon runners, ultramarathon participants, and triathletes sometimes develop hyponatremia during or after events. The primary cause is straightforward: they drink more fluid than they lose through sweat, breathing, and urination, leading to a net gain of water that dilutes blood sodium.
Several factors increase an athlete’s risk. Longer event durations give more time and opportunity to overdrink. Smaller body mass means a lower total blood volume, so the same amount of excess water produces a bigger dilution effect. The body also releases a hormone during prolonged exercise that tells the kidneys to retain water rather than excrete it, which compounds the problem. Over-the-counter anti-inflammatory painkillers, commonly taken during endurance events, can worsen this effect to a smaller degree.
Outside of athletics, people with certain kidney conditions, older adults on medications that affect fluid balance, and people with psychiatric conditions that involve compulsive water drinking are also at elevated risk. Infants are particularly vulnerable because of their tiny body size, which is why pediatricians warn against giving plain water to babies under six months.
Sports Drinks Don’t Fully Protect You
A common assumption is that drinking a sodium-containing sports drink instead of plain water will prevent overhydration problems. The reality is more nuanced. Some research shows that beverages with sodium can slow the decline in blood sodium levels during exercise. Even small amounts of sodium during prolonged activity in the heat help maintain more stable levels compared to plain water alone.
But here’s the critical point: sports drinks cannot prevent hyponatremia if you’re overdrinking. The amount of fluid you take in matters far more than what’s dissolved in it. Most sports drinks are still hypotonic, meaning they contain less sodium than your blood does. Drinking them in excess will still dilute your blood sodium. The most effective prevention strategy is simply not drinking more than you’re losing.
Practical Ways to Stay in Balance
Thirst is a reliable guide for most healthy adults in everyday life. Your body has a finely tuned system for signaling when it needs more fluid, and drinking when you’re thirsty, rather than forcing a specific number of glasses per day, keeps most people well within safe range.
During exercise, the best approach is to drink to match your losses rather than following a rigid schedule. Weigh yourself before and after a long workout or race: if you’ve gained weight, you drank too much. If you’ve lost a modest amount (up to about 2% of body weight), that’s normal and not harmful. Nausea, bloating, and a sloshing feeling in your stomach during activity are all signs to stop drinking until your body catches up.
If you’re participating in an endurance event lasting more than a few hours, pay attention to your body rather than drinking on a fixed timer. Smaller, more frequent sips matched to thirst are safer than large volumes consumed at aid stations out of habit or anxiety about dehydration. The goal is balance: enough fluid to support performance without tipping into the territory where your body can’t process what you’re taking in.