Overhydration is an excess of water in the body.
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 (see About Body Water). People, particularly athletes, who drink excessive water to avoid dehydration can develop overhydration, but usually do not. Often, people 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—see Hyponatremia (Low Level of Sodium in the Blood)), 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. Overhydration may also result from the syndrome of inappropriate secretion of antidiuretic hormone (see Syndrome of Inappropriate Secretion of Antidiuretic Hormone). 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.
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, and whenever dehydration is severe, 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.
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 signs of edema and doing blood tests to check the concentrations of electrolytes.
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.
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.
Last full review/revision January 2015 by James L. Lewis, III, MD