Diseases & Conditions



Dehydration is a deficiency of water in the body. Vomiting, diarrhea, excessive sweating, and use of diuretics may cause dehydration. People feel thirsty, and as dehydration worsens, they may sweat less and excrete less urine. If dehydration is severe, people may be confused or feel light-headed. Treatment is restoring lost water and electrolytes, usually by drinking but sometimes with intravenous fluids.

Dehydration occurs when the body loses more water than it takes in. Vomiting, diarrhea, the use of diuretics (drugs that increase urine excretion), profuse sweating (for example, during heat waves, particularly with prolonged exertion), and decreased water intake can lead to dehydration.

Dehydration is particularly common among older people, because their thirst center may not function as well as that in younger people. Therefore, some older people may not recognize that they are becoming dehydrated. Certain disorders such as diabetes mellitus (see Diabetes Mellitus (DM): Diabetes Mellitus ), diabetes insipidus (see Pituitary Gland Disorders: Central Diabetes Insipidus ), and Addison's disease (see Adrenal Gland Disorders: Addison's Disease ) can increase the excretion of urine and thereby lead to dehydration.

Dehydration is also common in infants and children because the amount of fluid lost during diarrhea or vomiting may represent a larger proportion of their body fluids than in older children and adults (see Problems in Infants and Very Young Children: Dehydration ).

Symptoms and Diagnosis

At first, dehydration stimulates the thirst center of the brain, causing people to drink more fluids. If water intake cannot keep up with water loss, dehydration becomes more severe. Sweating decreases, and less urine is excreted. Water moves from inside the cells to the bloodstream to maintain the needed amount of blood (blood volume) and blood pressure. If dehydration continues, tissues of the body begin to dry out, and cells begin to shrivel and malfunction. Symptoms of mild to moderate dehydration include thirst, reduced sweating, reduced skin elasticity, reduced urine production, and dry mouth. With severe dehydration, blood pressure can fall, causing light-headedness or faintness, particularly upon standing (a condition called orthostatic hypotension). If dehydration continues, shock and severe damage to internal organs, such as the kidneys, liver, and brain occur. Brain cells are particularly susceptible to more severe levels of dehydration. Consequently, confusion is one of the best indicators that dehydration has become severe. Very severe dehydration can lead to coma.

Dehydration can often be diagnosed because of the symptoms and the results of an examination. But sometimes doctors do blood tests for people who appear seriously ill or who take certain drugs or have certain disorders. Dehydration normally causes the sodium level in the bloodstream to increase (see Minerals and Electrolytes: Syndrome of Inappropriate Secretion of Antidiuretic Hormone ). The reason is that although the common causes of dehydration (such as profuse sweating, vomiting, and diarrhea) result in a loss of electrolytes (especially sodium and potassium), even more water is lost, so the concentration of sodium in the blood rises.

Spotlight on Aging

Older people are particularly susceptible to dehydration. In older people, common causes of dehydration include confusion and disorders that make obtaining fluids difficult (usually because of restricted mobility). Additionally, older people sense thirst more slowly and less intensely than younger people do, so those who are otherwise well may not drink enough fluids. Also, older people have a higher percentage of body fat. Because fat tissue contains less water than lean tissue, the total amount of water in the body tends to decrease with age. Also, because the kidneys excrete excess water less well, older people develop overhydration more easily than younger people.


Prevention is better than cure. Adults should drink at least 6 glasses of fluids daily (including fluid from eating foods high in water content, such as fruits and vegetables). Fluid intake should be increased on hot days and during or after prolonged exercise. Exercise, fever, and hot weather increase the body's need for water. Flavored sports drinks have been formulated to replace electrolytes lost during vigorous exercise. These drinks can be used to prevent dehydration. Before exercising, people with heart or kidney disorders should consult their doctors about how to safely replace fluids.


For treating mild dehydration, drinking plenty of water may be all that is needed. With moderate and severe dehydration, lost electrolytes (especially sodium and potassium) must be replaced. Oral rehydration solutions that contain appropriate amounts of electrolytes are available without a prescription. These solutions work well to treat mild dehydration, especially that caused by vomiting or diarrhea in children (see Problems in Infants and Very Young Children: Treating Dehydration ). Sports drinks do not necessarily contain enough electrolytes to be an adequate substitute for these solutions.

More severe dehydration requires treatment by doctors with intravenous solutions containing sodium chloride. The intravenous solution is given rapidly at first and then more slowly as the physical condition improves.

Treatment is also directed at the cause of dehydration. For example, when people have diarrhea, drugs to control or stop the diarrhea may be necessary.

Last full review/revision August 2008 by James L. Lewis, III, MD

Source: The Merck Manual Home Edition