Can You Drink Too Much Water? Hyponatremia Explained

Yes, you can, but for most healthy people it's rare. Drinking a very large amount of water in a short time, faster than your kidneys can get rid of it, can lower the sodium in your blood to a dangerous level, a condition called hyponatremia. Such cases are highly unusual and have involved some athletes and people with certain psychiatric disorders, and the main way to avoid it is simple: drink when you're thirsty, and don't drink far more than you lose.

What happens when you drink too much water

Your blood keeps its sodium within a narrow range. MedlinePlus, the US National Library of Medicine's health site, gives 136 to 144 mmol/L as normal. Hyponatremia means a level below 135 mmol/L, and the US National Academies note that symptoms usually don't appear unless it falls below 130 mmol/L.

Healthy kidneys are very good at getting rid of extra water, but they have a top speed. The National Academies put it at about 0.7 to 1.0 liters (about 24 to 34 fl oz) an hour. A 2017 review on exercise-associated hyponatremia gives a similar figure: 500–1,000 ml (about 17–34 fl oz) an hour through the kidneys. According to the National Academies, acute water intoxication has happened when people drank large amounts of fluid much faster than that.

The National Academies set no upper limit for water intake, because healthy people in temperate climates normally regulate how much they drink on their own.

How rare is it?

Very rare for most people. Announcing its report on water in 2004, the National Academies described drinking water in amounts that can be life-threatening as highly unusual, and mentioned some athletes and people with certain psychiatric disorders. For everyday life, the same announcement says "the vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide."

So if you drink when you're thirsty, with meals and through the day, you're not in the group this article is mainly about. For how much is typical in a day, see how much water should you drink a day.

Who is at risk

Endurance athletes

Exercise-associated hyponatremia (EAH) is low blood sodium that develops during physical activity or up to 24 hours afterward, according to an international consensus statement from 2015. The 2017 review names the most common causes as "overdrinking beyond thirst" together with the body holding on to water because of a hormone called vasopressin.

A common clue is weight gain. Most athletes who develop symptoms of EAH have gained weight during the event, and the review points out that such gains "can only occur through ingestion of fluids in excess of body losses." In other words, they drank more than they sweated out.

The same review lists things that can raise vasopressin and make the body hold on to water:

pain, emotion and exercise itself
nausea or vomiting
low blood sugar
heat
some medicines, such as anti-inflammatory painkillers (NSAIDs) and some antidepressants (SSRIs)

People with some medical conditions or medicines

Drinking isn't the only route to low sodium. MedlinePlus lists other causes, including:

diuretics ("water pills")
heart failure
kidney disease
cirrhosis of the liver
SIADH, a hormone problem that makes the body hold on to water
burns, diarrhea, sweating or vomiting

If any of these apply to you, the right amount to drink is a question for your doctor. If a doctor has told you to limit fluids (for example for heart failure or kidney disease), follow their limit rather than general advice.

Symptoms of low blood sodium

MedlinePlus lists these symptoms of hyponatremia:

confusion, irritability or restlessness
convulsions (seizures)
fatigue
headache
loss of appetite
muscle weakness, spasms or cramps
nausea and vomiting

"When your body's sodium level drops too much, it can be a life-threatening emergency," MedlinePlus warns. Several of these symptoms, like headache, nausea and tiredness, can have many other causes, so symptoms alone don't tell you what's going on. If someone becomes confused, has a seizure or passes out, especially during or after a long event where they drank a lot, call your local emergency number.

How to avoid drinking too much

You don't need to measure every sip. A few habits go a long way.

Day to day, let thirst guide you. For healthy people in everyday conditions, the National Academies say thirst does the job.
Spread drinks through the day. Sipping regularly keeps you well within what your kidneys can clear, unlike forcing down liters at once. It's also what Hydrolotl rewards: the axolotl's gills bloom when your drinks are spread across the day.
During exercise, don't drink more than you lose. The National Athletic Trainers' Association says active people should not drink more than their exercise-related losses, and should not gain weight during exercise. Weighing yourself before and after a long session is a simple check: if you're heavier afterward, you drank more than you lost.
Drink according to thirst in long events. That's the advice of the 2017 review, which says following thirst as a real-time guide "appears safe and effective." Not all experts agree; our guide to water during exercise covers the debate.
Treat sodium as individual. The National Athletic Trainers' Association says sodium supplementation before and during exercise should be individualized based on a person's own losses and needs.
Follow medical advice first. If you have a condition that affects your sodium or fluid balance, or you take medicines that do, your doctor's advice comes before any general rule.

For everyday checks on whether you're drinking enough, see what your urine color means.

FAQ

How much water is too much in a day?

There's no official daily maximum: the National Academies set no upper limit for water. What matters more is speed. Healthy kidneys can clear at most about 0.7 to 1.0 liters (about 24 to 34 fl oz) an hour, and drinking far faster than that has caused water intoxication.

What are the signs that you've drunk too much water?

Low blood sodium can cause headache, nausea and vomiting, fatigue, muscle cramps or weakness, and confusion or restlessness, and in severe cases seizures. During exercise, gaining weight is a warning sign that you've drunk more than you've lost.

Can drinking too much water kill you?

In rare cases, yes: severe low blood sodium can be a life-threatening emergency. But the National Academies describe dangerous overdrinking as highly unusual, and healthy people who drink when they're thirsty normally keep their water balance on their own.

Should you drink to thirst during a marathon?

The 2017 review on exercise-associated hyponatremia recommends drinking according to thirst, though not all experts agree. Either way, the National Athletic Trainers' Association's rule applies: don't drink more than you lose, so you don't finish heavier than you started.

Sources

This article is general information, not medical advice. If you have a health condition or your doctor has given you a fluid limit, follow their advice.