Dehydration is when a person's body doesn't have enough water and other fluids to function normally.
It is of particular concern in babies and young children, and in the elderly.
Mild to moderate dehydration can be prevented by drinking more fluids (rehydration) but severe dehydration, which can have serious consequences, requires prompt medical care.
Having sufficient water in our bodies is essential for the body to function properly. Fluid is lost from the body every day in the form of water vapour from the lungs when breathing, and in sweat, urine, and stools.
Small quantities of electrolytes (salts) are lost with fluid loss. Dehydration can result in an imbalance of electrolytes, especially sodium, potassium, and chloride, which affects the way the body functions.
Dehydration can result from not drinking enough fluid, losing too much fluid, or a combination of both. People might not drink enough because they:
- Do not feel like eating or drinking due to illness
- Are too busy and forget to drink
- Do not have immediate access to safe drinking water
- Have a sore throat or mouth sores.
People might lose body fluids more quickly than normal because of:
- Excessive sweating due to vigorous physical activity, especially in hot and humid weather
- Prolonged fever. The higher the fever, the greater the dehydration
- Severe vomiting and/or diarrhoea, which can result in substantial fluid and electrolyte loss in a short space of time
- Urinating too much, due to uncontrolled diabetes or certain medications (eg: diuretics and some blood pressure medications that cause more frequent urination).
Severe diarrhoea and vomiting is the most common cause of dehydration in babies and young children.
Signs and symptoms
Signs and symptoms of dehydration in adults include:
- Increased thirst
- Dry mouth and tongue
- Muscle cramps
- Dark coloured, strong-smelling urine
- Not urinating much and/or passing urine less often than normal.
Signs and symptoms of dehydration in babies and young children include:
- Dry mouth and tongue
- Fewer or no wet nappies
- Fewer or no tears when crying
- Sunken soft spot on their head (fontanelle)
- Sunken eyes and cheeks
Dehydration can usually be diagnosed on the basis of a person’s physical signs and symptoms. A doctor may also look for the following signs of dehydration:
- Low blood pressure
- Blood pressure that drops when standing up after lying down (postural hypotension)
- White fingertips that when pressed do not return to a pink colour
- Reduced skin "fullness" (when a dehydrated person's skin is pinched into a fold, it may slowly sag back into place. In a person who is hydrated the skin springs back more quickly)
- Rapid heartbeat.
The following laboratory tests may be performed to determine the severity of the dehydration and/or the cause of the dehydration:
- Blood tests to check electrolyte levels and kidney function
- Blood sugar test for diabetes
- Urine tests.
Left untreated, dehydration can lead to serious complications including:
- Heat exhaustion or heatstroke
- Urinary and kidney problems, including urinary tract infection and kidney stones
- Fits (seizures)
- Low blood volume shock (hypovolemic shock) which is a serious complication that can be life-threatening
- Brain damage
The primary aims of treatment for dehydration are to restore the body’s normal fluid volume and balance of electrolyte levels. To treat mild to moderate dehydration:
- Drink plenty of fluids such as water, diluted fruit juice, and sports drinks that contain electrolytes
- If frequent vomiting makes it difficult to keep water down, try drinking small amounts more frequently
- Avoid high-sugar drinks, and caffeinated and alcoholic beverages.
Babies and young children who are dehydrated should not be given substantial amounts of water. Too much water on its own can further dilute already low levels of electrolytes and lead to complications. Instead, they should be given diluted fruit juice or a rehydration solution (available from pharmacies).
A teaspoon or syringe may be helpful for giving fluids to babies. It is advisable to see your doctor early if dehydration is suspected in a young one, as it is often difficult to assess the hydration level accurately, and the condition may change very rapidly.
Severe dehydration requires a hospital visit and fluids and electrolytes given through a vein, ie: intravenously, for rapid rehydration and recovery.
Further information and support
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Cotter, J.D., et al. (2014) Are we being drowned in hydration advice? Thirsty for more? Extrem Physiol Med. 2014;3:18
Mayo Clinic (2019). Dehydration (Web Page). Rochester, MN: Mayo Foundation for Medical Education and Research. https://www.mayoclinic.org/diseases-conditions/dehydration/symptoms-causes/syc-20354086 [Accessed: 01/02/20]
MedlinePlus (2017). Dehydration (Web Page). Bethesda, MD: U.S. National Library of Medicine. https://medlineplus.gov/ency/article/000982.htm [Accessed: 01/02/20]
NHS (2019). Dehydration. Redditch: National Health Service (NHS) England. https://www.nhs.uk/conditions/dehydration/ [Accessed: 01/02/20]
O’Toole, M.T. (Ed.) (2017). Dehydration. Mosby’s Dictionary of Medicine, Nursing & Health Professions (10th ed.). St Louis, MI: Elsevier.
Last reviewed: February 2020