Dehydration in adults

Key points about dehydration in adults

  • Dehydration is the loss of water and salts from your body.
  • Feeling thirsty is a sign you need more fluid.
  • Most people experience mild dehydration at some time, but if it becomes severe it can lead to death.
  • Mild dehydration can be treated by drinking small amounts of water frequently. 
  • Babies, older adults and people who work outside are most at risk of severe dehydration. 
  • For severe dehydration seek urgent medical help or call Healthline 0800 611 116.
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For healthy people, the best way to know that you need more fluid is if you feel thirsty. 

In the early stages of dehydration (mild dehydration), symptoms include:

  • a dry, sticky mouth and tongue
  • feeling lightheaded or dizzy
  • weakness
  • a headache
  • feeling very tired – no energy.

As dehydration becomes worse (moderate or severe dehydration), symptoms include: 

  • extreme thirst
  • dry mouth and cracked lips
  • urinating (peeing) less
  • not peeing for 8 hours (a sign of severe dehydration)
  • dizziness when you stand up that doesn't go away after a few seconds
  • weakness
  • feeling sleepy, tired or confused
  • cramping in your arms and legs.
If you have signs of moderate or severe dehydration, see your doctor or an after-hours clinic immediately or call 111 for an ambulance.

Dehydration is when you don't have enough body fluids for your body to carry out normal functions easily. About 75% (three-quarters) of your body is water. Your bones are 22% water and your blood is nearly all (92%) water. Your body uses water to replace blood and other fluids so it can function properly. 

Along with water, your body also needs electrolytes. These are salts normally found in blood, other fluids and cells. When you are dehydrated, you also lose these salts.

You can survive without food for more than 30 days, but less than a week without water. 

Common causes of dehydration
  • Diarrhoea (runny poos) and vomiting (being sick).
  • Having a fever (high temperature).
  • Not eating or drinking while you’re sick.
  • Being outside in very hot conditions.
  • Not drinking enough before, during or after hard exercise or heavy work.
  • Some health conditions such as kidney disease or diabetes – these cause you to urinate (pee) more often.
  • Some medicines such as water pills (diuretics) – these can cause your body to lose water.
  • Even having a cold or sore throat – because you're less likely to feel like eating or drinking when you're sick. 

Although anyone can become dehydrated, those who become dehydrated most easily are babies under 1-year-old, older adults and people who work or exercise outside. 

Babies under 1 year old

Dehydration can quickly become serious in children. If you have a child under the age of 6 months who has vomiting/diarrhoea, or whom you suspect is dehydrated, take them to see a doctor right away. Young children often can't tell you that they're thirsty, nor can they get a drink for themselves. The younger the child, the easier it is for them to become dehydrated. Read more about dehydration in babies and children.

Older adults

As you age, your body's fluid reserve becomes smaller, your ability to retain water is reduced and your sense of thirst becomes less reliable. This is made worse by conditions such as diabetes and dementia, and by certain medicines. Older adults also may have mobility problems that limit their ability to get water easily.

People who work or exercise outside

When it's hot and humid, your risk of dehydration and heat illness increases. That's because when the air is humid, sweat can't evaporate and cool you as quickly as it normally does. This can lead to a higher body temperature and the need for more fluids. Read more about heat stroke and heat exhaustion.

  • Always drink plenty of fluids during the day, especially when working or exercising in the sun.
  • Where possible, try to do all physical outdoor activities in the cooler parts of the day.
  • Drink water before you play sport, during if possible, and after to ensure your body stays hydrated. 
  • Drink plenty of fluids if you have diarrhoea, vomiting or fever – see your doctor if you cannot keep fluids down.
  • Water is best. Avoid high-sugar, high-calorie drinks such as undiluted fruit juice, fizzy drinks and sports drinks. They are not as hydrating and drinking these regularly is a leading cause of obesity.
  • Avoid caffeinated and alcoholic drinks. These can make dehydration worse as they make you pee more.

Mild dehydration can be treated by drinking water frequently in small amounts until you are no longer thirsty and your urine (pee) is a light yellow colour. Even if you are vomiting, some of this water will still be absorbed by your body. 

If drinking water is not enough to treat the dehydration, your doctor or pharmacist may recommend electrolyte solutions such as Electral or Pedialyte. You can buy these from your pharmacist. Prepare them according to the instructions on the packet. Read more about oral rehydration solutions. 

Moderate or severe dehydration often needs treatment in hospital and may involve fluids being given by a nasogastric tube (a tube through your nose into your stomach) or intravenously (through a needle into a vein). 

How much water you need depends on many factors, including your health, how active you are and where you live.

It has been recommended we should drink 6 to 8 glasses of water a day. This is a good guide, although there is no research-based evidence to support this. A good guide is the colour of your urine (pee). It should be a very light-coloured yellow. If it is a deep yellow then it is likely you are not drinking enough water.


If you have kidney stones, drinking plenty of water each day can lower your chances of getting another stone. You should also drink extra amounts of water when experiencing any dehydrating conditions (such as hot, humid weather, high altitudes or physical exertion). 

5 easy ways to drink more water

  • Carry a water bottle with you.
  • Set an alarm or download an app to remind you when to drink more fluids.
  • Have a glass of water before and after each meal.
  • Replace other drinks with water, especially drinks with sugar, caffeine or alcohol in them.
  • In winter, drink warm water with lemon, honey or mint leaves.

Read more about drinking the right amount of water.


Image credit: NHS, UK





Learn more

Dehydration(external link)(external link) NHS, UK
Dehydration(external link)(external link) MedlinePlus, US

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Credits: Healthify editorial team. Healthify is brought to you by Health Navigator Charitable Trust.

Reviewed by: Dr Alice Miller, FRNZCGP

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