Drinking too much water can dilute the sodium in your blood to dangerous levels, a condition called hyponatremia. While rare in everyday life, it can cause symptoms ranging from nausea and headaches to seizures, coma, and in extreme cases, death. Most documented cases involve consuming more than 5 liters (about 1.3 gallons) within a few hours.
What Happens Inside Your Body
Your blood normally contains sodium at a concentration between 135 and 145 milliequivalents per liter. When you drink water faster than your kidneys can excrete it, that sodium concentration drops. Water then moves into your cells through osmosis, causing them to swell.
Most cells in your body can tolerate some swelling. Brain cells cannot. Your skull is rigid, so when brain tissue expands, pressure builds quickly. This increased pressure is what drives the most serious symptoms of water intoxication, from confusion and drowsiness to seizures and loss of consciousness. How fast sodium drops matters enormously: a gradual decline over days produces milder symptoms, while a rapid drop over hours can cause life-threatening brain swelling.
Early and Late Warning Signs
The first symptoms of overhydration tend to feel like something you’d brush off. Nausea, a dull headache, and mild fatigue are common early signs. You might also notice muscle cramps or a general sense of restlessness.
As sodium levels fall further, the symptoms escalate: vomiting, confusion, significant drowsiness, and irritability. At the most severe end, people experience seizures, lose consciousness, or slip into a coma. The progression from mild to life-threatening can happen within hours if a large volume of water is consumed rapidly. A 27-year-old soldier was hospitalized with weakness and drowsiness after drinking roughly 6 quarts of water in two hours during training.
How Much Water Is Too Much
There’s no single number that applies to everyone, because kidney capacity, body size, and sweat rate all matter. Healthy kidneys can process roughly 0.8 to 1 liter of water per hour. Problems tend to arise when intake significantly outpaces that rate. A review of water intoxication cases found that most involved more than 5 liters consumed over just a few hours, with some cases involving 10 to 20 liters.
For context, most healthy adults need about 11.5 cups (2.7 liters) to 15.5 cups (3.7 liters) of total fluid per day from all sources, including food. That’s spread across an entire day. Trouble starts when people try to consume large amounts in a short window, whether from misguided hydration advice, a dare, or anxiety about dehydration during exercise.
Who Is Most at Risk
Endurance athletes are the group most commonly affected. Severe hyponatremia has been documented in marathon runners, triathletes, ultramarathon participants, and people on long desert hikes. For years, athletes were told to drink as much fluid as possible during exercise. As that advice spread, the incidence of exercise-related hyponatremia appeared to increase, particularly in the United States. The problem has also been reported in less obvious settings: team sports, yoga, weightlifting, rowing, tennis, and even musical theater rehearsals.
Certain medications also raise the risk by interfering with how your kidneys handle water. Some common antidepressants (particularly SSRIs), opioid painkillers, anti-seizure medications, anti-inflammatory drugs like ibuprofen, and antipsychotic medications can all trigger your body to retain more water than normal. They do this by increasing the activity of a hormone that tells your kidneys to hold onto fluid instead of excreting it. If you take any of these medications and are planning heavy exercise or a situation where you’ll be drinking a lot of fluids, it’s worth being aware of the added risk.
People with certain underlying conditions are also more vulnerable. Kidney disease, heart failure, and some cancers and lung conditions can impair the body’s ability to regulate water balance. Smaller body size matters too, because it takes less excess water to shift sodium levels in a lighter person.
Sports Drinks Don’t Solve the Problem
A common assumption is that switching from water to a sports drink will protect you from hyponatremia because of the added electrolytes. It doesn’t. A study of marathon runners conducted by Harvard-based researchers found that 13% had some degree of hyponatremia after the race, and it was equally common among those who drank sports drinks and those who drank plain water. The sodium content in commercial sports drinks is simply too low to offset the dilution caused by drinking excessive volumes of fluid. The real fix is drinking less, not drinking something different.
How to Drink Safely During Exercise
The current best practice for athletes is to drink to thirst rather than on a schedule. Earlier advice to “stay ahead of your thirst” contributed to the rise in exercise-related hyponatremia. Your thirst mechanism is well calibrated for most situations. During long events, paying attention to your body’s signals is more protective than forcing fluid at regular intervals.
A practical approach: weigh yourself before and after exercise a few times to get a sense of your personal sweat rate. If you’re not losing significant weight during a workout, you’re drinking enough. Gaining weight during exercise is a red flag that you’re taking in more fluid than you’re losing.
What Treatment Looks Like
Mild overhydration often resolves on its own once you stop drinking and your kidneys catch up. You may feel nauseated or foggy for a few hours, but your body will restore its sodium balance through normal urine output.
Severe cases are a medical emergency. In the hospital, treatment focuses on carefully raising sodium levels back to a safe range using concentrated salt solutions delivered intravenously. The tricky part is that correcting sodium too quickly carries its own risk: it can damage the protective coating around nerve fibers in the brain. So doctors raise levels gradually while monitoring blood work frequently. Recovery depends on how low sodium dropped and how quickly treatment started. People who reach the hospital before seizures or loss of consciousness generally recover fully.
If you or someone around you develops confusion, vomiting, or seizures after drinking large amounts of water, that warrants emergency care. Time matters, because brain swelling can progress rapidly once it starts.