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Dehydration ˌdē-hī-ˈdrā-shən

by James L. Lewis, III, MD

Dehydration is a deficiency of water in the body.

  • Vomiting, diarrhea, excessive sweating, and use of diuretics may cause dehydration.




Dehydration occurs when the body loses more water than it takes in. Vomiting, diarrhea, the use of drugs that increase urine excretion (diuretics), 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), diabetes insipidus (see Central Diabetes Insipidus), and Addison disease (see Addison Disease) can increase the excretion of urine and thereby lead to dehydration.

Dehydration is also common among 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 Dehydration in Children).



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—see Dizziness or Light-Headedness When Standing Up). 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.


  • A doctor's evaluation

  • Sometimes blood tests


Dehydration normally causes the sodium level in the blood to increase (see Hypernatremia (High Level of Sodium in the Blood)). 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.




  • Replacing fluids and electrolytes


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

Mild dehydration

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 Treating Dehydration). Sports drinks do not necessarily contain enough electrolytes to be an adequate substitute for these solutions. However, when a person is vomiting, drinking fluids may not be sufficient to treat dehydration.

Severe dehydration


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