Dehydration - Introduction

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
a 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
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 dehydrated, drink plenty of fluids such as water, diluted squash or fruit juice. These are much more effective than large amounts of tea or coffee. Fizzy drinks may contain more sugar than you need and may be harder to take in large amounts.

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

Infants and small children who are dehydrated shouldn't be given large amounts of water alone as the main replacement fluid. This is because it can dilute the already low level of minerals in their body too much and lead to other problems.

Instead, they should be given diluted squash or a rehydration solution (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.

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.