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Water Intoxication: the risk of drinking too much water

How to avoid overhydration

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First published: 11.Oct.2018

Overview: How much water is too much?

Drinking too much water (or fluids) is bad for your health. Excessive intake of water beyond what your thirst level tells you can upset your body's delicate balance of fluids and electrolytes.

Overhydration can provoke a serious health condition known as "water intoxication" or "water poisoning", which can have significant consequences, even death.

In this article, we will learn how much water is too much, the causes, amount, and levels required to overhydrate, and how to prevent it.

In this Article (Index)

glass filling with water
Listen to your body, don't over hydrate

What Is Water Intoxication?

Water intoxication is a serious life-threatening condition caused by ingesting too much water.

Is water toxic?

Lack of adequate water intake can lead to dehydration, but overdoing it can cause a condition known as water poisoning, water intoxication, or water toxemia.

Water is one of the safest and least toxic chemical compounds known but in excess, it can be dangerous.

Toxicity is measured with an index known as "Lethal Dose 50%" or LD50. This is the quantity required of a given substance to kill 50% of the test population. It is usually tested in rats and the value is given as a value in milliliters or milligrams per kilogram of body weight.

The LD50 of water is above 90 g ⁄ kg in rats (roughly 1.5 oz per lb). In contrast, the LD50 in rats for ethanol (alcohol) is 0.007 g ⁄ kg, table salt is 3 g⁄kg and sugar is 30 g ⁄ kg.

Despite its low toxicity, if the quantity ingested overpowers the body's capacity to process it, water intoxication occurs.

The human body has a very delicate balance of electrolytes (charged atoms or molecules, also known as "ions") which allow it to function correctly.

Electrolytes are formed when the minerals that we consume are dissolved in the body fluids. For instance, salt dissociates into sodium and chlorine ions. Other critical electrolytes are bicarbonate, potassium, magnesium, and calcium.

Our body needs a stable balance of electrolytes to maintain its processes working healthyly.

Drinking too much water upsets the delicate balance of electrolytes (and not drinking enough water also disrupts this balance!).

The trigger: drinking too much water

But when a large quantity of pure water is ingested, the kidneys can't eliminate it fast enough, so water builds up in the blood and seeps into the spaces between the body's cells.

This process dilutes the concentration of electrolytes in the body's extracellular fluids or ECF.

Extracellular fluid (ECF) comprises all the fluids that are outside of our cells, about one-third of the total body fluids is of the ECF type: It is the fluid in the blood plasma, the fluid located in the tiny spaces between cells (interstitial fluid), the cerebrospinal fluid and gastrointestinal fluid.


The dilution of electrolytes affects a critical one: Sodium

The condition is known as "hyponatremia", a word that combines the Greek word " hypo " = "under", the Latin word " natrium " = "sodium" and the Greek word " haima " = "blood". It means "low sodium concentration in blood plasma".

Hyponatremia occurs when sodium levels drop below 135μmol/liter.

This condition is quite common during marathons, where many participants finish the races with hyponatremia.

Hyponatremia is also frequent in hospitalized patients with a history of chronic alcoholism.

Dilution of Extracellular electrolytes

As ECF electrolyte concentration decreases, osmosis kicks in, shunting fluids into the cells.

Osmosis is the movement of a solvent (in this case water) through a permeable membrane (such as the cell's walls). The solvent flows from a region with a low solute concentration (in this case the ECF, whose ions are more diluted) to a region of high solute concentration (the cells, whose electrolyte balance has, until now, not changed).

Swelling of the cells a danger signal

As water moves into the cells it increases the intracellular fluid volume, swelling the cells and at the same time, diluting their elecrtolytes.

This cellular swelling can cause pulmonary edema (fluid and swelling in the lungs), headache (due to brain edema).

Brain edema is very dangerous because as the neurons uptake water, they swell causing the brain to expand inside the skull. This builds pressure and damages the brain.

This causes dizziness, vomiting, disorientation, seizures, odd behavior, and unconsciousness (coma). Death may follow.

Causes of overhydration

The most common cause of overhydration is drinking too much pure water during strenuous exercise or work, to replace fluids lost due to sweating. This is quite common in the following circumstances:

  • Marathon runners
  • Overexertion
  • Heat stress

Water drinking competitions are also rare events that provoke water toxemia. It can also appear in those who overhydrate while using drugs such as MDMA ("Ecstasy"), to counter the drug's dehydrating effect.

Sports and outdoor activities are risk factor

Hew-Butler (2017) reported deaths that occurred due to water intoxication in the following contexts (1):

  • High school football players following practice.
  • A soldier on the first day of Ranger training.
  • A policeman participating in a 12 mi (19 km) bike ride.
  • A college student performing calisthenics for a fraternity.
  • A person who was trekking.
  • An ironman triathlete.
  • A canoeist during an ultradistance race.

Overhydrating is a risk in all sorts of sporting activities

Some other activities where overzealous fluid intake has caused water intoxication are: long-distance swimming, yoga, and mountain cycling.

Routine screenings at sports events detected asymptomatic hyponatremia in one third of rugby players after a match, 70% of a team of junior rowers after a long training session, 11% of Ironman triathletes (after a race), and 67% of ultramarathon runners tested during a race. (2).

woman long distance runner
Sports can make you drink too much water

Why does strenuous exercise trigger water toxemia?

The hypothalamus -located deep inside our brain- produces a hormone called vasopressin which regulates how the kidneys eliminate fluids. It is known as the antidiuretic hormone.

Its function is to conserve the body's water during periods of intense physical activity by reducing the amount of water that the kidneys remove from body fluids.

A normal person's kidneys can remove around 0.8 to 1 quart (0.8 - 1 liter) of water per hour; so a "normal" hydration level of 0.8 - 1 quart ⁄ hour will be eliminated without any risk of hyperhydration.

But during extreme physical activity such as a marathon, vasopressin can decrease the kidneys' excretion rate from 0.8 quarts per hour to values as low as 0.1 quarts per hour. So if the marathonist hydrates at a normal rate of 0.8 quarts per hour, the fluid will build up in his or her body at 0.7 quart per hour!

Children are also at risk

During hot spells or heat waves, the authorities recommend increasing the intake of fluids, especially water to avoid dehydration. However, parents should be aware that there is a risk of water intoxication in children for the following reasons (3):

  • Their kidneys and renal function are not yet fully mature.
  • They have a strong thirst drive: so they will drink all the fluids that they are given.

To make matters worse, as the symptoms of dehydration and overhydration are very similar, one has to be very careful to avoid worsening the problem by overhydrating an already overhydrated child.

Tip: if the child is dehydrated their diapers will be drier as they will urinate less frequently.


The drug ecstasy provokes thirst and also increases the production of the antidiuretic hormone, meaning that those using the drug will drink more fluids and urinate less. This builds up fluids leading to water intoxication and can cause death.

Preventing water intoxication

The best option is to keep a balance of input and output of fluids: you should try to drink as much as you are losing (if you are sweating 0.5 liters an hour drink 0.5 liters of fluid to replace the loss).

But, how do you measure sweat? You could weigh yourself and estimate your loss of fluids, but the best solution is to drink until your thirst is satisfied: avoid drinking beyond that limit.

Treating overhydration

Seek medical assistance immediately. Therapy will consist of limiting water intake, administrating osmotic diuretics, the possible application of hyperosmolar solutions (if brain edema develops).

Garigan (1999) reported a case where a soldier's water intoxication symptoms were mistaken as those of dehydration and heat injury, which led to oral hydration as a treatment, which caused brain and pulmonary edema followed by death (4).

Closing comments

Overdoing it with your water intake is as bad as dehydrating. Listen to your body, be aware of your thirst.

Take-home point

Keep hydrated but do not overdo it. Drink until your thirst is gone.

Read More

> > Do I need 8 Glasses of Water a Day?

The myth of the "8 x 8" rule: learn why it is not true.

About this Article

Water intoxication, A. Whittall

©2018, 11 Oct. 2018. Updated. 19 Oct. 2020.

Tags: dehydration, water, weight loss, overhydration, hydration

References and Further Reading

(1) Hew-Butler T, Loi V, Pani A, Rosner MH, (2017). Exercise-Associated Hyponatremia: 2017 Update, Front Med (Lausanne). 2017 Mar 3;4:21. doi: 10.3389/fmed.2017.00021

(2) Siegel AJ (2015). Fatal water intoxication and cardiac arrest in runners during marathons: prevention and treatment based on validated clinical paradigms, Am J Med. 2015 Oct;128(10):1070-5. doi: 10.1016/j.amjmed.2015.03.031

(3) R M Kayani and P Ramnarayan, (2007). Water intoxication and the heat wave, Arch Dis Child. 2007 Jan; 92(1): 90-91. doi: 10.1136/adc.2006.107599

(4) Garigan TP1, Ristedt DE, (1999). Death from hyponatremia as a result of acute water intoxication in an Army basic trainee, Mil Med. 1999 Mar;164(3):234-8

Mom of 2 dies of water intoxication, family says. Good Morning America, Aug. 4, 2023

About this Article

Water Intoxication: the risk of drinking too much water, A. Whittall

©2023, 01 Sept. 2023. Update scheduled for 01 Sept. 2025.

Tags: hydration, dehydration, overhydration, water

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