Page last reviewed: 13/07/2011

Water makes up over two-thirds of the healthy human body. It is essential for lubricating the joints and eyes, aiding digestion, flushing out waste and toxins and keeping skin healthy.

Dehydration occurs when the normal water content of your body is reduced, upsetting the delicate balance of minerals (salts and sugar) in your body fluid. Many of your body's cells depend on these minerals being maintained at the correct levels to function properly.

Some of the early warning signs that you are dehydrated are feeling thirsty and lightheaded and having concentrated, strong-smelling urine (see Symptoms of dehydration). The body works less efficiently with even a relatively low level of fluid loss.

How it happens

Dehydration is generally caused by not drinking enough fluid or by losing fluid and not replacing it.

You can lose fluid through vomiting or diarrhoea (for example, if you have gastroenteritis), through sweating (for example, if you have a fever or are exercising in hot conditions) or through urinating (for example, if you have diabetes).

Drinking too much alcohol can also cause dehydration, which is the main symptom of a hangover.

Types of dehydration

There are two types of dehydration:

  • Isotonic dehydration is when you lose water and salt in the same proportion as the water and salt in the fluid surrounding your cells. It is the type of dehydration most often caused by diarrhoea.
  • Hypernatraemic dehydration usually happens in infants or children. 'Hypernatraemic' means high levels of salt in the blood, so hypernatraemic dehydration is when a child loses relatively more water than salt, for example when they have watery diarrhoea or excessive vomiting. 

At-risk groups

People who are particularly at risk from dehydration are:

  • babies and infants, because they have a low body weight and are sensitive to even small amounts of fluid loss
  • older people, because they may be less aware of becoming dehydrated and the need to keep drinking fluids
  • people with a long-term condition, such as diabetes or alcoholism
  • athletes, because of the amount they sweat


Mild dehydration carries few risks and can usually be easily treated by replacing lost fluids (see Treating dehydration).

However, the effects of severe or untreated dehydration can be serious and sometimes life threatening.

How much should we drink?

In Ireland, you should drink about 1.2 litres (six to eight glasses) of fluid every day to prevent dehydration. You need more in hotter climates.

It is fine to have drinks that contain caffeine, such as tea, coffee and cola, but they are mild diuretics, which means they make you urinate more. Drink these alongside non-caffeine drinks and increase your intake of water if you experience any signs of dehydration.

If you are working hard in hot conditions, you need to drink more fluid than you normally would. It is recommended that you drink about 250ml (half a pint) of water every 15 minutes or 500ml (a pint) every 30 minutes.

Page last reviewed: 13/07/2011

Dehydration occurs when there is a 1% or greater reduction in body weight due to fluid loss.

Depending on the percentage of body weight lost, dehydration can be described as mild, moderate or severe.

Mild to moderate dehydration

The first sign of dehydration is thirst. Other symptoms are:

  • dizziness or light headedness
  • headache
  • tiredness
  • dry mouth, lips and eyes
  • dark, concentrated urine
  • passing only small amounts of urine infrequently (fewer than three or four times a day)

Moderate dehydration (a 3-5% decrease in body weight due to fluid loss) causes you to lose strength and stamina and is the primary cause of heat exhaustion.

If dehydration is chronic (ongoing), it can affect kidney function and may lead to the development of kidney stones. It can also cause:

  • harm to your liver, joints and muscles
  • cholesterol problems
  • constipation

Severe dehydration

Severe dehydration is a decrease of more than 5% of body weight due to fluid loss.

As well as severe thirst, you may also have:

  • dry, wrinkled skin
  • an inability to urinate
  • irritability
  • sunken eyes
  • low blood pressure (hypotension)
  • a weak pulse
  • a rapid heartbeat
  • cool hands and feet
  • seizures
  • a low level of consciousness

If body weight is reduced by more than 10%, this is extremely serious. If it is not treated immediately, this level of dehydration can lead to death as the blood stops circulating. You may need to go to hospital and be put on a drip to restore the substantial loss of fluids.

Spotting dehydration in babies

Look out for:

  • a sunken soft spot (fontanelle) on their head
  • few or no tears when they cry
  • dry mouth
  • fewer wet nappies
  • drowsiness
  • fast breathing

Page last reviewed: 13/07/2011

Dehydration is caused by not drinking enough fluid or by losing fluid (through sweat, tears, vomiting, urine or diarrhoea) and not replacing it.

The severity of your dehydration can often depend on a number of factors, such as the climate, your level of physical activity and your diet.


Dehydration often results from an illness, such as gastroenteritis, where you lose fluid through persistent diarrhoea and vomiting.

Diarrhoea usually occurs when fluid cannot be absorbed from your bowel contents or when your body secretes fluid into your bowel, causing watery stools. See the A-Z topic on Diarrhoea for more information.


You can also become dehydrated after sweating excessively from a fever, heavy work in hot conditions or heatstroke.


Dehydration can also occur from drinking too much alcohol. Alcohol is a diuretic, which means it causes you to urinate more.

The headache associated with a hangover indicates that your body is dehydrated. This is why it is important to drink plenty of water when you have been drinking alcohol.


If you have diabetes, you are at risk of dehydration because you have high levels of glucose in your bloodstream. Your kidneys try to get rid of this glucose by creating more urine, so your body becomes dehydrated from urinating more frequently. For more information, see Diabetes.

Page last reviewed: 13/07/2011

A good indicator of dehydration is how often you pass urine and how concentrated it is. If you urinate fewer than three or four times a day, the amount of urine is small and it is unusually dark in colour, you are probably dehydrated.

The best way to assess the severity of dehydration is to measure changes in body mass. Blood and urine tests can also indicate the extent of dehydration.

When to see your GP

If you experience persistent symptoms of dehydration despite drinking plenty of fluids, see your GP. They will carry out a physical examination and look at the dryness of your skin.

If dehydration is suspected, you may be given a blood or urine test to check the balance of sodium and potassium (salts) in your body.

Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
Chronic usually means a condition that continues for a long time or keeps coming back.
Dehydration is an excessive loss of fluids and minerals from the body.
Diarrhoea is the passing of frequent watery stools when you go to the toilet.
A high temperature, also known as a fever, is when someone's body temperature is 38C (100.4F) or above.
Urine test
Urinalysis or UA is when a urine sample is tested, commonly to check for any signs of infection or protein or sugar levels.
Vomiting is when you bring up the contents of your stomach through your mouth.

Measuring dehydration

If you lose 1% or more of your body weight due to fluid loss, you are dehydrated. Depending on the percentage of body weight lost, dehydration can be mild, moderate or severe.

Page last reviewed: 13/07/2011

Generally, to treat dehydration you have to rehydrate the body by drinking plenty of fluids, such as water, diluted squash, diluted fruit juice or semi-skimmed milk.

A sweet drink can be useful for replacing lost sugar and a salty snack, such as a packet of crisps, can help replace lost salt.

Dehydrated infants and children should not be given water as the main replacement fluid as this can further dilute the minerals in their body and make the problem worse. Give them diluted squash, diluted fruit juice or a special rehydration solution (see below).

If you find it hard to hold down fluids because you are vomiting or have diarrhoea, take smaller amounts more frequently. Give your child small amounts using a spoon or syringe.

Rehydration solutions

If you are dehydrated, you will have lost sugar and salts as well as water. You should, therefore, drink a rehydration solution that contains all the essential ingredients that you need to re-establish the right balance of body fluids. The solution should contain a mixture of potassium and sodium salts, as well as glucose or starch.

Several different rehydration products are available from pharmacies or on prescription from your GP, including solutions suitable for infants and children. Ask your GP or pharmacist for advice.

Severe dehydration

If you are severely dehydrated, you may need to go to hospital. Fluid may be given by a nasogastric tube (a feeding tube inserted through the nose and passed down into the stomach) or saline drip (into a vein).

If you have had bowel surgery, some rehydration solutions may not contain enough salt to restore hydration. In this case, you will need a higher-strength solution. Your GP or surgeon can recommend a suitable rehydration solution.

Babies, infants and elderly people need urgent treatment if they are dehydrated.

A drip is used to pass fluid or blood into your bloodstream through a plastic tube and needle that goes into one of your arteries or veins.
Dehydration is an excessive loss of fluids and minerals from the body.
Veins are blood vessels that carry blood from the rest of the body back to the heart.


In rare cases, it is possible to become overly hydrated while exercising. This condition, known as hyponatremia, is caused by low levels of sodium (salt) in the blood and can occur when too much water is drunk in a very short time. The condition sometimes affects endurance athletes whose blood sodium is reduced through sweat and then further diluted by drinking large amounts of water. Typical symptoms of hyponatremia include nausea, vomiting and headache. In the most serious cases the brain swells, causing confusion, seizures, coma and even death.

Page last reviewed: 13/07/2011

You can avoid becoming dehydrated by drinking plenty of fluids. As well as water, the following drinks can help to avoid dehydration:

  • semi-skimmed milk
  • diluted fruit juice
  • diluted squash

If you are particularly active or if the weather is particularly hot, there is a greater risk that you will become dehydrated. To prevent dehydration, increase your fluid intake. Also increase your fluid intake if you are ill with sickness or diarrhoea.

When exercising, drink up to one litre (two pints) of fluid each hour of exercise on top of your normal daily amount. This should be increased if you are exercising in warm conditions as you will sweat more and fluid will be lost from your body more rapidly.

How much should children drink?

There are no recommendations for the amount of water or other fluids children need. However, like adults, to stay healthy it is important for children to replace the water they lose to prevent dehydration. It is also important to remember that children will lose more water in hotter climates and when they have been physically active.

Drinks for children

Try to give your child healthier drinks as part of a healthy, balanced diet.

Sugary drinks, such as some fizzy drinks and squash, contain very few nutrients. The added sugar they contain can also damage teeth. Children who have lots of sugary drinks are more likely to put on weight and be overweight.

Fruit juice contains vitamins that are good for children's health. However, it also contains a kind of sugar that can damage teeth, so it is best to only drink fruit juice at mealtimes.

Water and milk are the best types of drinks to give children, especially between meals, and they do not cause tooth decay.

Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.
The brain controls thought, memory and emotion. It sends messages to the body controlling movement, speech and senses.
A coma is a sleep-like state when someone is unconscious for a long period of time.
Dehydration is an excessive loss of fluids and minerals from the body.
Diarrhoea is the passing of frequent watery stools when you go to the toilet.
Nausea is when you feel like you are going to be sick.
Vomiting is when you bring up the contents of your stomach through your mouth.

Content provided by NHS Choices and adapted for Ireland by the Health A-Z.

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