Dehydration occurs when your body loses more fluid than you take in.

When the normal water content of your body is reduced, it upsets the balance of minerals (salts and sugar) in your body, which affects the way it functions.

Water makes up over two-thirds of the healthy human body. It lubricates the joints and eyes, aids digestion, flushes out waste and toxins, and keeps the skin healthy.

Some of the early warning signs of dehydration include:

  • feeling thirsty and lightheaded
  • dry mouth
  • tiredness
  • having dark coloured, strong-smelling urine
  • passing urine less often than usual

A baby may be dehydrated if they:

  • have a sunken soft spot (fontanelle) on their head
  • have few or no tears when they cry
  • have fewer wet nappies (nappies will feel lighter)
  • are drowsy

The body is affected even when you lose a small amount of fluid.

Read more about the symptoms of dehydration.

What causes dehydration?

Dehydration is usually caused by not drinking enough fluid to replace what we lose. The climate, the amount of physical exercise you are doing (particularly in hot weather) and your diet can contribute to dehydration.

You can also become dehydrated as a result of an illness, such as persistent vomiting and diarrhoea, or sweating from a fever.

Read more about the causes of dehydration.

Who is at risk from dehydration?

Anyone can become dehydrated, but certain groups are particularly at risk. These include:

  • babies and infants - they have a low body weight and are sensitive to even small amounts of fluid loss
  • older people - they may be less aware that they are becoming dehydrated and need to keep drinking fluids
  • people with a long-term health condition - such as diabetes or alcoholism
  • athletes - they can lose a large amount of body fluid through sweat when exercising for long periods 

What to do


If you're at risk of dehydration, drink plenty of fluids such as water or diluted squash. These are much more effective than large amounts of tea, coffee or fizzy drinks, which contain caffeine.

If you're finding it difficult to keep water down because you're vomiting, try drinking small amounts more frequently.

If you are dehydrated, you will need rehydration fluids (available from pharmacies).

Children and infants

Continue to breastfeed infants at risk of dehydration, or give them other milk feeds. Older children at risk of dehydration can be given diluted squash.

Avoid giving fruit juices and carbonated drinks. 

If an infant or child is already dehydrated, they should be given rehydration fluids (available from pharmacies). You might find a teaspoon or syringe can be helpful for getting fluid into a young child.

If left untreated, severe dehydration can be serious and cause fits (seizures), brain damage and death.

Read more about treating dehydration.

When to see your GP

See your GP if your symptoms continue, despite drinking plenty of fluids, or if you think your baby or toddler is dehydrated.

If your GP suspects dehydration, you may have a blood test or a urine test to check the balance of salts (sodium and potassium) in your body.

Contact your GP, out-of-hours service or NHS 111 straight away if you have any of the following symptoms:

  • extreme thirst
  • feeling unusually tired (lethargic) or confused
  • not passing urine for eight hours
  • rapid heartbeat
  • dizziness when you stand up that doesn't go away after a few seconds

You should also contact your GP if your baby has had six or more episodes of diarrhoea in the past 24 hours, or if they have vomited three times or more in the past 24 hours. 

Symptoms of dehydration

Symptoms of dehydration

Dehydration can be mild, moderate or severe, depending on how much of your body weight is lost through fluids.

Two early signs of dehydration are thirst and dark-coloured urine. This is the body's way of trying to increase water intake and decrease water loss.

Other symptoms may include:

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

Dehydration can also lead to a loss of strength and stamina. It's a main cause of heat exhaustion.

You should be able to reverse dehydration at this stage by drinking more fluids.

If dehydration is ongoing (chronic), it can affect your kidney function and increase the risk of kidney stones. It can also lead to muscle damage and constipation.

When to see your GP

See your GP if your symptoms continue despite drinking fluids, or if you suspect that your baby or toddler is dehydrated.

You should also contact your GP if your baby has had six or more episodes of diarrhoea in the past 24 hours, or if they have vomited three times or more in the past 24 hours.

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

Severe dehydration

If dehydration is left untreated, it can become severe.

Severe dehydration is a medical emergency and requires immediate medical attention.

Contact your GP, out-of-hours service or NHS 111 straight away if you have any of the following symptoms:

  • feeling unusually tired (lethargic) or confused, and you think you may be dehydrated
  • dizziness when you stand up that doesn't go away after a few seconds
  • not passing urine for eight hours
  • a weak pulse
  • a rapid pulse
  • fits (seizures)
  • a low level of conciousness

If severe dehydration is not treated immediately, it can lead to complications. This level of dehydration needs hospital treatment and you will be put on a drip to restore the substantial loss of fluids.

Dehydration in babies

A baby may be dehydrated if they have:

  • a sunken soft spot (fontanelle) on their head
  • few or no tears when they cry
  • a dry mouth
  • fewer wet nappies
  • dark yellow urine
  • drowsiness
  • fast breathing
  • cold and blotchy-looking hands and feet

Read about how to treat dehydration in babies.

Causes of dehydration

Causes of dehydration

Dehydration is caused by not drinking enough fluid or by losing more fluid than you take in. Fluid is lost through sweat, tears, vomiting, urine or diarrhoea.

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

There are several causes of dehydration, which are described below.


Dehydration is often the result of an illness, such as gastroenteritis, where fluid is lost through persistent bouts of diarrhoea and vomiting.


You can also become dehydrated if you sweat excessively after a fever, exercise, or carrying out heavy, manual work in hot conditions.

In these situations, it's important to drink regularly to replace lost fluids. It doesn't necessarily need to be hot for you to lose a significant amount of fluid from sweating.

Children and teenagers are particularly at risk because they may ignore the symptoms of dehydration, or not know how to recognise and treat them.


Dehydration can also occur as a result of drinking too much alcohol. Alcohol is a diuretic, which means it makes you wee more.

The headache associated with a hangover indicates that your body is dehydrated. You should try to drink plenty of water when you have been drinking alcohol.


If you have diabetes, you're at risk of becoming dehydrated because you have high levels of glucose in your bloodstream. Your kidneys will try to get rid of the glucose by creating more urine, so your body becomes dehydrated from going to the toilet more frequently.

Read more about the different types of diabetes.

Who's at risk?

The groups of people most at risk of dehydration are:

  • babies and infants - their low body weight makes them sensitive to even small amounts of fluid loss
  • older people - they may be less aware they're becoming dehydrated and need to drink fluids
  • people with a long-term health condition - such as diabetes or alcoholism
  • athletes - they can lose a large amount of body fluid through sweat when exercising for long periods


It's possible to become overhydrated while exercising. This is known as hyponatremia and it's caused by low sodium (salt) levels in the blood. It can occur if too much water is drunk over a short period of time.

Hyponatremia sometimes affects athletes whose blood sodium level is reduced through sweat and then diluted by drinking large amounts of water.

Symptoms of hyponatremia include nausea, vomiting and headache. In serious cases, the brain can swell, causing confusion, seizures, coma and, in rare cases, death.

Treating dehydration

Treating dehydration

The only way to treat dehydration is to rehydrate the body with rehydration fluids, available from pharmacies.


If your baby is dehydrated, take them to see your GP as soon as possible. They'll be able to recommend an oral rehydration solution, described below.

Giving your baby regular sips (a few times an hour) of oral rehydration solution in addition to their usual feed (breastmilk, formula milk and water) will help to replace lost fluids, salts and sugars.

Avoid giving your baby fruit juice, particularly if they have diarrhoea and vomiting, because it can make it worse.

Infants and children

Infants and children who are dehydrated should also take an oral rehydration solution.

If your child is finding it difficult to hold down fluids because of vomiting, give them smaller amounts more frequently. You may find it easier to use a spoon or a syringe.

Read more about vomiting in adults and vomiting in children and babies.

Oral rehydration solutions

When you're dehydrated, you lose sugar and salts, as well as water. Drinking a rehydration solution will enable you 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.

There are several different rehydration products available over the counter from pharmacies or on prescription from your GP, including solutions that are suitable for infants and children.

Ask your GP or pharmacist for advice about the most suitable rehydration solution for you or your child.

Severe dehydration

Seek immediate medical help if you suspect someone is severely dehydrated (see symptoms of severe dehydration).

They may need to be admitted to hospital for treatment. In particular, babies, infants and elderly people will need urgent treatment if they become dehydrated.

Fluid may be given up the nose using a nasogastric tube or using a saline drip into a vein (intravenously). This will provide essential nutrients faster than using solutions that you drink.

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

Preventing dehydration

Preventing dehydration

You should drink plenty of fluids to avoid becoming dehydrated.

Most of the time, you can prevent dehydration by drinking water regularly throughout the day. Be guided by your thirst, but be aware that in hot weather, when exercising and during illness, you should drink more.

Mild dehydration can be relieved by drinking more water and diluted fruit squash. If necessary, you can purchase oral rehydration solutions (ORS) from a pharmacy. As a guide, passing pale or clear-coloured urine (wee) is a good sign that you're well hydrated.

Drink regularly

If you're active, or if the weather is particularly hot, there's a greater risk that you will become dehydrated. To prevent becoming dehydrated, you should increase your fluid intake.

As different people sweat at different rates, it's very difficult to provide specific recommendations about how much fluid you should drink. However, you should drink more than normal while exercising, and it's particularly important to keep well hydrated if you're exercising in warm conditions. This is because you will sweat more and fluid will be lost from your body more rapidly.

Rarely, drinking more fluid than your body can process can reduce the amount of sodium (salt) in your blood. This can lead to a serious and potentially fatal condition called hyponatremia. If you start to feel discomfort and bloating from drinking, stop drinking and allow time to recover.


If you, your child or someone you are caring for is ill, particularly with a fever, vomiting or diarrhoea, there's a high risk of becoming dehydrated, so it's important to start replacing fluid as soon as possible.

Advice for children

There are no specific recommendations regarding the amount of water or other fluids that children need.

However, it's important for children to replace lost fluid, to prevent dehydration. Like adults, children lose more water when they are in hotter climates and when they are physically active.

You should give your child healthy drinks as part of an overall healthy, balanced diet.

See the page on drinks and cups for more information and advice on specific drinks for young children.