Water is not bad for you. It’s essential for every function in your body, from regulating temperature to filtering waste through your kidneys. But like almost anything, water can cause harm in specific circumstances: when you drink far too much too quickly, when contaminants are present in your supply, or when certain medications change how your body handles fluids. Understanding those edge cases is useful, but the bottom line is that most people would benefit from drinking more water, not less.
How Much Water You Actually Need
The general guideline for healthy adults is about 11.5 cups (2.7 liters) per day for women and 15.5 cups (3.7 liters) per day for men. That total includes all fluids and the water content in food, which typically accounts for about 20% of your daily intake. So you don’t need to drink that entire amount from a glass.
Your actual needs shift based on activity level, climate, body size, and whether you’re pregnant or breastfeeding. If you’re sweating heavily during exercise or in hot weather, you’ll need more. If you eat a lot of fruits and vegetables (which are mostly water), you’ll need less from beverages. Thirst is a reliable guide for most healthy people, though it becomes less sensitive as you age.
When Too Much Water Becomes Dangerous
Water intoxication is real, but it’s rare and requires drinking large amounts in a short window. Your kidneys can process roughly 800 to 1,000 milliliters per hour under normal conditions, with a peak urine flow rate of about 10 to 15 milliliters per minute. If you consistently pour in more water than your kidneys can expel, the excess dilutes the sodium in your blood, a condition called hyponatremia.
Sodium keeps fluid balanced between the inside and outside of your cells. When blood sodium drops too low, water moves into cells by osmosis, causing them to swell. Most cells can tolerate some swelling, but brain cells are confined inside the skull. That’s what makes severe hyponatremia life-threatening: the brain has very little room to expand. Your brain does have a defense mechanism, actively pumping out certain molecules to restore balance and limit swelling, but it can be overwhelmed if sodium drops too fast.
Hyponatremia is classified by severity. A blood sodium level of 130 to 134 milliequivalents per liter is mild, 125 to 129 is moderate, and anything below 125 is severe. Mild cases may cause no symptoms at all. Severe cases can progress to seizures, coma, and death.
Early Signs You’ve Had Too Much
The first symptoms of overhydration are easy to dismiss: nausea, a bloated stomach, and a headache. If you notice these after drinking a lot of water in a short time, stop drinking and let your body catch up. More concerning signs include drowsiness, muscle weakness or cramps, confusion, irritability, and swelling in your hands or feet. These indicate your sodium levels have dropped enough to affect how your cells function.
If symptoms progress to confusion, severe headache, or disorientation, that’s a medical emergency. Untreated severe water intoxication can lead to seizures and loss of consciousness.
Why Endurance Athletes Are at Higher Risk
Marathon runners, ultramarathon competitors, and other endurance athletes face a surprisingly high risk of exercise-related hyponatremia. During prolonged exercise, two things work against you: you tend to drink large volumes of fluid to stay hydrated, and your body loses sodium through sweat. At the same time, exercise triggers hormonal changes that reduce urine output, meaning your kidneys temporarily lose their ability to flush excess water efficiently.
The numbers are striking. In a study of runners completing a 246-kilometer ultramarathon, 65% developed hyponatremia by the end of the race. Even at the 93-kilometer checkpoint, 23% already had low sodium levels. Among finishers, 22% had moderate to severe hyponatremia, with sodium dropping below 130. Risk factors include slower pace (more time drinking on course), higher body mass index, and drinking plain water rather than fluids containing electrolytes.
This doesn’t mean casual joggers need to worry. The risk scales with duration and intensity. But if you’re training for events lasting several hours, replacing electrolytes alongside water matters more than just maximizing fluid intake.
Medications That Change the Equation
Certain medications impair your body’s ability to excrete water or maintain sodium levels, making it easier to tip into hyponatremia even with normal water intake. The most common culprits are thiazide diuretics (often prescribed for blood pressure), which are the leading drug-related cause of low sodium. The risk is highest in the first few weeks after starting the medication, then gradually decreases over about three months.
Antidepressants in the SSRI class, antipsychotic medications, anti-seizure drugs, and proton pump inhibitors (used for acid reflux) can also trigger hyponatremia, typically shortly after you begin taking them. If you’re on any of these medications and notice symptoms like persistent nausea, confusion, or unusual fatigue, it’s worth having your sodium levels checked. You don’t necessarily need to restrict water, but you should be aware that your threshold for overhydration may be lower than average.
What’s Actually in Your Water
The other way water can potentially harm you has nothing to do with quantity. It’s about what’s dissolved in it. U.S. tap water is regulated by the Environmental Protection Agency, which sets maximum contaminant levels for dozens of substances. For most people on municipal water systems, the supply is safe. But “safe” has limits, and those limits have tightened in recent years as scientists learn more about long-term exposure.
Lead is one of the most well-known concerns. The EPA’s goal for lead in drinking water is zero, with an action level of 0.010 milligrams per liter. Lead contamination typically comes from aging pipes and plumbing fixtures rather than the water source itself. If more than 10% of tap water samples in a system exceed the action level, the utility must take corrective steps. Older homes, particularly those built before 1986, are more likely to have lead solder or service lines.
PFAS, sometimes called “forever chemicals” because they don’t break down in the environment, have received increasing attention. The EPA now regulates individual PFAS compounds at extremely low levels. For PFOA and PFOS, the two most studied varieties, the maximum allowable concentration is 4 parts per trillion, an extraordinarily strict limit. Several other PFAS compounds are capped at 10 parts per trillion. These standards were finalized recently and water systems are still working toward compliance.
Arsenic, which occurs naturally in groundwater in some regions, has a maximum contaminant level of 0.010 milligrams per liter. Private wells are not covered by EPA regulations, so if your home uses well water, periodic testing is the only way to know what’s in it.
The Practical Takeaway on Hydration
For the vast majority of people, water is one of the best things you can put in your body. Dehydration is a far more common problem than overhydration. The risk of water intoxication applies mainly to people drinking extreme volumes in a short period, endurance athletes replacing sweat with plain water for hours on end, and individuals on medications that affect sodium balance. If you’re drinking when you’re thirsty, spreading your intake throughout the day, and not forcing yourself to hit some arbitrary gallon-a-day target, your kidneys are well equipped to handle the job.