Dehydration occurs when you use or lose more fluid than you take in, and your body doesn't have enough water and other fluids to carry out its normal functions. If you don't replace lost fluids, you will get dehydrated.

Common causes of dehydration include vigorous exercise, especially in hot weather; intense diarrhea; vomiting; fever or excessive sweating. Not drinking enough water during exercise or in hot weather even if you're not exercising also may cause dehydration. Anyone may become dehydrated, but young children, older adults and people with chronic illnesses are most at risk.

You can usually reverse mild to moderate dehydration by drinking more fluids, but severe dehydration needs immediate medical treatment. The safest approach is preventing dehydration in the first place. Keep an eye on how much fluid you lose during hot weather, illness or exercise, and drink enough liquids to replace what you've lost.

Symptoms Causes Risk factors Complications Prevention

Mild to moderate dehydration is likely to cause:

  • Dry, sticky mouth
  • Sleepiness or tiredness — children are likely to be less active than usual
  • Thirst
  • Decreased urine output
  • No wet diapers for three hours for infants
  • Few or no tears when crying
  • Dry skin
  • Headache
  • Constipation
  • Dizziness or lightheadedness

Severe dehydration, a medical emergency, can cause:

  • Extreme thirst
  • Extreme fussiness or sleepiness in infants and children; irritability and confusion in adults
  • Very dry mouth, skin and mucous membranes
  • Little or no urination — any urine that is produced will be darker than normal
  • Sunken eyes
  • Shriveled and dry skin that lacks elasticity and doesn't "bounce back" when pinched into a fold
  • In infants, sunken fontanels — the soft spots on the top of a baby's head
  • Low blood pressure
  • Rapid heartbeat
  • Rapid breathing
  • No tears when crying
  • Fever
  • In the most serious cases, delirium or unconsciousness

Unfortunately, thirst isn't always a reliable gauge of the body's need for water, especially in children and older adults. A better indicator is the color of your urine: Clear or light-colored urine means you're well hydrated, whereas a dark yellow or amber color usually signals dehydration.

When to see a doctor

If you're a healthy adult, you can usually treat mild to moderate dehydration by drinking more fluids, such as water or a sports drink (Gatorade, Powerade, others). Get immediate medical care if you develop severe signs and symptoms such as extreme thirst, a lack of urination, shriveled skin, dizziness and confusion.

Treat children and older adults with greater caution. Call your family doctor right away if your loved one:

  • Develops severe diarrhea, with or without vomiting or fever
  • Has bloody or black stool
  • Has had moderate diarrhea for 24 hours or more
  • Can't keep down fluids
  • Is irritable or disoriented and much sleepier or less active than usual
  • Has any of the signs or symptoms of mild or moderate dehydration

Go to the nearest hospital emergency room or call 911 or your emergency medical number if you think a child or older adult is severely dehydrated. You can help prevent dehydration from becoming severe by carefully monitoring someone who is sick and giving fluids, such as an oral rehydration solution (CeraLyte, Pedialyte, others), at the first sign of diarrhea, vomiting or fever and by encouraging children to drink plenty of water before, during and after exercise.

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