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Dehydration - symptoms, prevention

Dehydration is the lack of sufficient water in our body tissues, which occurs when more water is lost from the body than is taken in, causing a range of symptoms. 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.    

General information

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 is also known as hypohydration.


Dehydration occurs when the loss of fluids from the body is not matched by the intake of fluids. Therefore, 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.

Risk factors

Although dehydration can affect anyone, certain groups are at higher risk of dehydration:

  • Babies who, due to their low body weight and precarious electrolyte balance, are sensitive to even small amounts of fluid loss   
  • Older people who have reduced awareness of becoming dehydrated and a lower volume of fluid in their bodies. They are also more likely to have medical conditions or be taking medications that increase their risk of dehydration
  • People with a cold, sore throat or influenza (flu) which makes them less likely to feel like eating or drinking
  • People with gastroenteritis e.g: norovirus, rotavirus
  • People with chronic (long-term) medical conditions, eg: diabetes, kidney disease
  • People with outdoor occupations, who have greater exposure to heat and humidity
  • Athletes, who can lose a large volume of fluid through sweat when exercising for extended periods of time.

Signs and symptoms

Signs and symptoms of dehydration in adults include:

  • Increased thirst
  • Light-headedness
  • Headache
  • Dry mouth and tongue
  • Lethargy/tiredness
  • Muscle cramps
  • Confusion
  • 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
  • Listlessness
  • Irritability.


It is usually possible for dehydration to 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 finger tips that when pressed do not return to a pink colour
  • Reduced skin turgor (fullness or tension caused by fluid content of tissues). When the skin of a person who is dehydrated is pinched into a fold, it may slowly sag back into place. In a person who is hydrated, however, the skin springs back immediately.    
  • Rapid heart beat.

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
  • Fits (seizures)
  • Low blood volume shock (hypovolemic shock)
  • Brain damage
  • Death.


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, 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.


There is no standard recommendation for daily fluid intake because fluid requirements are different for different people and vary with age, climate, and level of physical activity. Passing urine that is light-yellow coloured is a good sign of adequate hydration.

Also, thirst is not always a reliable early indicator of dehydration. In some people, particularly older adults, dehydration precedes feeling thirsty. It is important, therefore, to increase water intake during hot weather or when ill. Dehydration can be prevented by:

  • Drinking plenty of fluids every day, especially when the weather is hot or when exercising
  • Eating foods with high water content, eg: fruits and vegetables
  • Drinking plenty of fluids, including rehydration solutions if fever, vomiting, or diarrhoea is present. Do not wait for signs of dehydration.
  • Drinking extra fluids when feeling unwell due to a short-term illness, eg: cold, influenza, bronchitis.

People with kidney stones should drink plenty of fluid every day to lower the chances of developing another stone.

Further information and support

Free phone: 0800 611 116

Free phone: 0800 933 922


Cotter, J.D., et al. (2014) Are we being drowned in hydration advice? Thirsty for more? Extrem Physiol Med. 2014;3:18
Mayo Clinic (2016). Dehydration (Web Page). Rochester, NY: Mayo Foundation for Medical Education and Research. [Accessed: 29/04/17]
MedlinePlus (2015). Dehydration (Web Page). Bethesda, MD: U.S. National Library of Medicine. [Accessed: 29/04/17]
NHS Choices (2015). Dehydration. Redditch: National Health Service (NHS)
England. [Accessed: 29/04/17]
O’Toole, M.T. (Ed.) (2013). Dehydration. Mosby’s Dictionary of Medicine, Nursing & Health Professions (9th ed.). St Louis, MI: Elsevier Mosby.

Created: April 2017


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