All the cells and organs in your body need water to function properly.
Because of this, you need to constantly replace water lost through sweat, urine and breath.
Not drinking enough water can cause symptoms such as fatigue, dizziness and muscle cramps.
Conversely, drinking way too much water can also cause a serious condition called water intoxication.
What is water intoxication?
Also known as water poisoning, water intoxication is the disruption of brain function due to drinking too much water (1).
Drinking a lot of water increases the amount of water in your blood.
This water can dilute the electrolytes in your blood, especially sodium. When sodium levels fall below 135 mmol/L, it is called hyponatremia.
Sodium helps balance fluids between the inside and outside of cells.
When sodium levels drop due to excess water consumption, fluids shifts from the outside to the inside of cells, causing them to swell (2).
When this happens to brain cells, it can produce dangerous and potentially life-threatening effects.
Bottom line: Water intoxication results from drinking too much water. The excess water dilutes blood sodium levels and causes fluids to move inside cells, which then swell.
Drinking too much water can be dangerous
Water intoxication results from the swelling of cells.
When brain cells swell, pressure inside the skull increases. This pressure causes the first symptoms of water intoxication, which include:
Severe cases can produce more serious symptoms, such as:
- Increased blood pressure.
- Double vision.
- Difficulty breathing.
- Muscle weakness and cramping.
- Inability to identify sensory information.
Bottom line: Drinking too much water increases pressure inside the skull. This can cause various symptoms and even be fatal in severe cases.
Water intoxication has caused death
It's very difficult to consume too much water by accident, yet there have been reported cases of deaths due to this condition.
One report concerned 17 soldiers who developed hyponatremia as a result of excess water intake. Their blood sodium levels ranged from 115 to 130 mmol/L, but the normal range is 135-145 mmol/L (4).
Another report described how three soldiers died due to hyponatremia and cerebral edema. These deaths were associated with drinking 2.5-5.6 gallons (10-20 liters) of water in just a few hours (5).
The symptoms of hyponatremia can be misinterpreted as those of dehydration. One soldier, who was misdiagnosed as suffering from dehydration and heat stroke, died from water intoxication as the result of repeated oral hydration (3).
Water intoxication also occurs during sports, especially endurance sports. Over-hydration is common in these activities as a means to avoid dehydration.
At the 2002 Boston Marathon, 13% of participants had hyponatremia symptoms. 0.06% showed critical hyponatremia, with sodium levels less than 120 mmol/L (8).
Unfortunately, some instances of water intoxication at these sports events have resulted in deaths.
One study of 27 schizophrenics that had died young showed that five of them died due to self-induced water intoxication (13).
Bottom line: Water intoxication is most common among soldiers, endurance sports athletes and schizophrenia patients. Several hyponatremia cases and deaths have been reported in these populations.
How much water is too much?
Over-hydration and water intoxication happens when you drink more water than your kidneys can get rid of via urine.
But the amount of water isn't the only factor. How long you take to drink the water also counts.
You have a greater risk of developing water intoxication if you drink a lot of water in a short period of time. The risk is less if you drink the same amount over a much longer period of time.
Symptoms of hyponatremia can occur from as little as 0.8-1 gallons (3-4 liters) of water in a short amount of time (14).
Therefore, in order to avoid hyponatremia symptoms, you should not drink more than 27-33 ounces (0.8-1.0 liters) of water per hour, on average (14).
Many reported cases of water intoxication result from drinking large amounts of water in a short period of time.
For example, one report describes soldiers who developed symptoms after consuming half a gallon of water (1.8 liters or more) per hour (4).
Another report shows the development of hyponatremia with water intake of 2.5-5.6 gallons, or 10-20 liters, in just a few hours (5).
A case of water intoxication and prolonged hyponatremia also occurred in a healthy, 22-year-old male prisoner after he drank 1.5 gallons (6 liters) of water in 3 hours (1).
Finally, a 9-year old girl who drank almost a gallon (a total of 3.6 liters) of water in 1-2 hours developed water intoxication (14).
Bottom line: The kidneys are capable of excreting up to 7 gallons (28 liters) of fluid per day. However, they cannot excrete more than 1 liter per hour. Therefore, drinking more than this is not a good idea.
How much water do you actually need?
There is no specific number for how much water you actually need to drink a day. It differs for each individual.
To determine how much you need, consider your body weight, physical activity level and climate.
The Institute of Medicine (IOM) suggests the adequate water intake per day for men is 125 ounces (3.7 liters), while for women it is 91 ounces (2.7 liters).
However, these recommendations include water from beverages and foods (16).
A good rule of thumb is to listen to your body and drink when you feel thirsty. This should be enough to maintain good hydration levels.
However, relying on thirst alone may not work for everyone. Athletes, older adults and pregnant women may need to drink some extra water each day.