Merck Manual

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James L. Lewis III

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

Reviewed/Revised Apr 2022 | Modified Sep 2022
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Dehydration is a deficiency of water in the body.

  • Vomiting, diarrhea, excessive sweating, burns, kidney failure, 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 mineral salts (such as sodium and potassium) that are dissolved in the blood (electrolytes), usually by drinking but sometimes with intravenous fluids.

Symptoms of Dehydration

At first, dehydration stimulates the thirst center of the brain, causing thirst, a powerful motivator for people to drink more fluids. If water intake does not 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 (see About Body Water About Body Water Water accounts for about one half to two thirds of an average person’s weight. Fat tissue has a lower percentage of water than lean tissue and women tend to have more fat, so the percentage... read more ). 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

  • Dry mouth

In severe dehydration, the sensation of thirst may actually decrease and blood pressure can fall, causing light-headedness or fainting, particularly upon standing (a condition called orthostatic hypotension Dizziness or Light-Headedness When Standing Up In some people, particularly older people, blood pressure drops excessively when they sit or stand up (a condition called orthostatic or postural hypotension). Symptoms of faintness, light-headedness... read more ). If dehydration continues, shock Shock Shock is a life-threatening condition in which blood flow to the organs is low, decreasing delivery of oxygen and thus causing organ damage and sometimes death. Blood pressure is usually low... read more 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 and death.

Spotlight on Aging: Water Balance


Older people are particularly susceptible to dehydration. In older people, common causes of dehydration include

Additionally, older people sense thirst more slowly and less intensely than younger people do, so even those who are otherwise well may not drink enough fluids for a variety of reasons, including incontinence Urinary Incontinence in Adults Urinary incontinence is involuntary loss of urine. Incontinence can occur in both men and women at any age, but it is more common among women and older adults, affecting about 30% of older women... read more Urinary Incontinence in Adults or the fear of incontinence.

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.


In overhydration Overhydration 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... read more , the body contains too much water. In older people, the kidneys excrete excess water less efficiently, and thus older people can develop overhydration more easily than younger people do. Swelling (edema) may or may not occur.

Diagnosis of Dehydration

  • A doctor's evaluation

  • Sometimes blood tests

Dehydration can often be diagnosed from symptoms and the results of a doctor's examination. But sometimes doctors do blood tests for people who appear seriously ill or who take certain drugs or have certain disorders. For people who require more monitoring or testing in an emergency department or intensive care unit, doctors sometimes use ultrasound or special catheters to measure the severity of dehydration.

Prevention of Dehydration

Prevention of dehydration 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, when working or exercising in hot weather, during or after prolonged exercise, and, if possible, when people have vomiting and/or diarrhea.

Exercise, a high body temperature, 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. People should drink fluids with electrolytes before and during vigorous exercise as well as afterward. Before exercising, people with heart or kidney disorders should consult their doctors about how to safely replace fluids.

People should make sure that older family members have access to plenty of water when they are alone in a hot building or place.

Treatment of Dehydration

  • Replacing fluids and electrolytes

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 also be replaced.

Oral rehydration solutions Treatment Dehydration is loss of water from the body, usually caused by vomiting and/or diarrhea. Dehydration occurs when there is significant loss of body water and, to varying amounts, electrolytes... read more that contain appropriate amounts of electrolytes are available without a prescription. These solutions work well to treat dehydration, especially that caused by vomiting or diarrhea in children. Sports drinks do not necessarily contain enough electrolytes to be an adequate substitute for these solutions.

People who are vomiting may not be able to hold down enough fluid to treat dehydration. More severe dehydration requires treatment by doctors with intravenous solutions containing sodium chloride (salt). The intravenous solution is given rapidly at first and then more slowly as the physical condition improves.

Drugs Mentioned In This Article

Generic Name Select Brand Names
4-Way Saline, Adsorbonac, Altamist, Ayr Allergy & Sinus, Ayr Baby Saline, Ayr Saline Nasal, BD Posiflush Normal Saline, BD Posiflush Sterile Field Normal Saline, BD Posiflush SureScrub Normal Saline, Blairex Broncho Saline, Breathe Free Saline, Deep Sea , Entsol, HyperSal, Hyper-Sal, Hypertears, Little Remedies for Noses, Little Remedies Stuffy Nose, Monoject Sodium Chloride, Muro 128, NebuSal , Ocean, Ocean Complete, Ocean For Kids, Pediamist, PULMOSAL, Rhinaris, Rhinaris Lubricating, Saljet , Saljet Rinse, SaltAire, Sea Soft, Trichotine, Wound Wash, XYNASE, ZARBEE'S Soothing Saline Nasal Mist
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