Merck Manual

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James L. Lewis, III

, MD, Brookwood Baptist Health and Saint Vincent’s Ascension Health, Birmingham

Last full review/revision Jun 2020| Content last modified Jun 2020
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Overhydration is an excess of water in the body.

  • People can develop overhydration if they have a disorder that decreases the body’s ability to excrete water or increases the body's tendency to retain water.

  • Drinking too much water rarely causes overhydration because normal kidneys easily excrete excess water.

  • Often, no symptoms occur, but in severe overhydration, people may become confused or have seizures.

  • Fluid intake is restricted, and diuretics may be given.

(See also About Body Water.)

Overhydration occurs when the body takes in more water than it loses.

Overhydration can occur when people drink much more water than their body needs. People, particularly athletes, who drink excessive water to avoid dehydration can develop overhydration. People may also drink excessive water because of a psychiatric disorder called psychogenic polydipsia. The result is too much water and not enough sodium. Thus, overhydration generally results in low sodium levels in the blood (hyponatremia), which can be dangerous. However, drinking large amounts of water usually does not cause overhydration if the pituitary gland, kidneys, liver, and heart are functioning normally. To exceed the body’s ability to excrete water, a young adult with normal kidney function would have to drink more than 6 gallons of water a day on a regular basis.

Overhydration is much more common among people whose kidneys do not excrete urine normally—for example, among people with a disorder of the heart, kidneys, or liver, or among premature infants, whose kidneys are immature. Certain drugs, such as some antidepressants, can also cause overhydration in susceptible people.

Overhydration may also result from the syndrome of inappropriate antidiuretic hormone secretion. In this syndrome, the pituitary gland secretes too much vasopressin (also called antidiuretic hormone), stimulating the kidneys to conserve water when that is not needed.

Did You Know...

  • Drinking too much water without replacing salt can be harmful, occasionally even in healthy people.


Brain cells are particularly susceptible to overhydration and to low sodium levels in the blood. When overhydration occurs slowly and is mild or moderate, brain cells have time to adapt, so only mild symptoms (if any) like distractibility and lethargy may ensue. When overhydration occurs quickly, vomiting and trouble with balance develop. If overhydration worsens, confusion, seizures, or coma may develop.

When overhydration occurs and blood volume is normal, the excess water usually moves into the cells, and tissue swelling (edema) does not occur. When excess blood volume occurs, fluid can accumulate in the lungs and lower legs.


  • A doctor's examination

  • Blood and urine tests

Doctors try to distinguish between overhydration (too much water) and excess fluid in the blood (too much salt and increased blood volume) by examining the person for weight gain and signs of edema and doing blood and urine tests to check the concentrations of electrolytes.


  • Restriction of fluid intake

  • Treatment of the cause of overhydration

Regardless of the cause of overhydration, fluid intake usually must be restricted (but only as advised by doctors). Drinking less than a quart of fluids a day usually results in improvement over several days. If overhydration occurs with excess blood volume because of heart, liver, or kidney disease, restricting the intake of sodium is also helpful because sodium causes the body to retain water.

Drugs that tend to cause overhydration are stopped. Sometimes, doctors prescribe diuretics to increase the excretion of sodium and water in the urine. Other types of drugs also can increase water excretion and are sometimes used to treat overhydration when blood volume is normal. These drugs are generally used when a person is in the hospital and can be carefully monitored.

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