Water helps your body regulate temperature, move nutrients around, remove waste and keep your circulation working normally. But when you lose more fluid than you replace, dehydration can begin to affect how you feel.
It may start with thirst, darker urine or a dry mouth. More significant fluid loss can leave you dizzy, unusually tired or unwell, and serious dehydration needs urgent medical treatment. This guide explains the signs, common causes, sensible ways to replace fluids and when to get help in the UK.
Medical information checked: 22 October 2026. The guidance here is general; people with prescribed fluid restrictions or specific medical conditions should follow their clinician’s advice.
What is dehydration?
Dehydration occurs when your body loses more fluid than you take in. Everyone loses fluid through urine, sweat, breathing and bowel movements. Usually, food and drinks replace what is lost.
Not drinking enough is one cause, but dehydration can develop faster during a fever, hot weather, intense physical activity or illness causing vomiting and diarrhoea. Some medicines, particularly diuretics, can increase fluid loss. Babies, young children and older people may be at greater risk.
The NHS dehydration guide explains the symptoms, first steps and signs of more serious illness.
Early signs you may not be drinking enough
Look for several signs together rather than judging hydration from one symptom alone:
- Feeling thirsty: an everyday signal that your body needs fluid.
- Darker yellow, strong-smelling urine: the kidneys may be conserving water.
- Urinating less often: less fluid may be leaving the body.
- Dry mouth, lips or tongue: a common symptom, although it may have other causes.
- Headache or light-headedness: possible symptoms, especially during heat or illness.
- Unusual tiredness: dehydration is one possible explanation among many.
Urine colour is a useful everyday clue, not a medical test. Foods, supplements, some medicines and health conditions can change its appearance.
| What you notice | What it may suggest | Reasonable next step |
|---|---|---|
| Thirst and darker urine after being busy | You may need more fluid | Drink regularly and check whether symptoms improve |
| Headache and tiredness after time in the heat | Fluid loss could be contributing | Move somewhere cooler and replace fluids; assess other symptoms |
| Vomiting or diarrhoea | Water and salts may be lost | Take frequent small sips and ask a pharmacist about oral rehydration solutions |
| Dizziness on standing that does not go away | Possible more serious dehydration | Urgent GP advice or NHS 111 |
| Confusion or difficulty waking with signs of dehydration | Possible medical emergency | Call 999 or go to A&E |
1. Why does your urine become darker?
Your kidneys help manage the amount of water and dissolved substances in your body. When fluid is scarce, the body can conserve water, producing smaller quantities of more concentrated urine. This is why dehydration can make urine appear dark yellow and smell stronger.
The NHS advice on drinks and hydration suggests aiming for urine that is a clear, pale yellow colour. Very dark urine, particularly alongside dizziness or reduced urination, can be a warning sign. Urine colour alone cannot establish the cause.
2. Why can dehydration cause a headache?
Headache is among the symptoms the NHS lists for dehydration. It may be noticed after heavy sweating, illness or not drinking enough, but headaches also have many other causes, from migraine to infections and sleep problems.
If a headache happens alongside thirst or darker urine, addressing fluid loss may help. Do not assume that drinking water will treat every headache. A sudden, extremely severe headache or a headache with serious neurological symptoms needs emergency assessment. See our related guide: Why Do We Get Headaches? Common Causes Explained.
3. Why might you feel tired or dizzy?
Water helps maintain normal circulation. As fluid loss becomes more significant, you may feel weak, unusually tired or light-headed, particularly on standing.
These symptoms are not specific to dehydration. Anaemia, medicines, infections and many other problems can cause tiredness or dizziness. If your symptoms continue despite sensible fluid replacement, or you have signs of serious dehydration, seek medical advice. Dizziness when standing that does not go away is a reason to seek urgent help through a GP or NHS 111, according to the NHS.
4. How does dehydration affect your temperature?
Your body uses sweating to help release heat. Sweat evaporating from the skin provides cooling, but it also removes fluid. In hot conditions or during long periods of activity, losses can increase substantially.
Drink regularly when working or exercising in the heat, take breaks and move somewhere cooler if you become unwell. Dehydration is not the same thing as heatstroke: symptoms of severe heat-related illness can be an emergency even if someone has been drinking water.
Remember that dehydration is possible in winter too. You continue to lose fluid through ordinary body processes regardless of the weather.
5. What happens during vomiting or diarrhoea?
Illness involving repeated vomiting or diarrhoea can remove water as well as salts and minerals. If you feel sick, the NHS recommends starting with small sips and gradually drinking more as tolerated.
A pharmacist can recommend an oral rehydration solution (ORS) to replace fluid and electrolytes lost through illness. These products need to be mixed according to their instructions. They are different from ordinary sports or energy drinks, and extra sugar is not necessarily helpful.
Seek advice if you or a child cannot keep fluids down or continue to show signs of dehydration after using an appropriate oral rehydration solution. Read NHS guidance on diarrhoea and vomiting for further advice.
How much should you drink every day?
There is no exact number of glasses that suits everyone. The UK Eatwell Guide recommends six to eight cups or glasses of fluid a day as a general starting point. You may need more if you are active for long periods, living or working in hot conditions, pregnant, breastfeeding or recovering from illness.
This is a guideline for fluid, not a requirement to drink eight glasses of plain water in addition to other drinks. The NHS counts water, lower-fat milk and lower-sugar or sugar-free drinks, including tea and coffee, toward daily fluid intake. Food also supplies some water.
The amount you need is affected by your health and circumstances. If you have been told to follow a restricted fluid allowance—for example because of certain kidney or heart conditions—follow that personalised advice rather than increasing your intake to meet a general target.
Do tea, coffee and foods count?
Yes. Ordinary tea and coffee contribute fluid, although the NHS recommends consuming caffeine in moderation. Water is an inexpensive, sugar-free everyday option, and other suitable drinks can add variety.
Fruit, vegetables, soup and yoghurt also provide fluid. However, sugary drinks should not automatically be your first choice for daily hydration. The NHS recommends limiting fruit juice and smoothies to a combined total of 150ml per day because of their sugar content.
People often focus on how many bottles of water they finish. A more useful habit is to drink regularly, notice thirst and pay attention to the wider context—exercise, illness, weather and any medical advice you have received.
Can drinking too much water be dangerous?
Yes. Consuming extremely large amounts of water in a short period can disturb the body’s balance of salts, including sodium. In rare situations this contributes to hyponatraemia, which can be serious.
The goal is appropriate hydration, not forcing yourself to drink as much as possible. Long endurance exercise, particular health conditions or prescribed fluid limits may require individual advice about fluids and electrolytes.
Dehydration in babies, children and older adults
Babies and young children can become dehydrated quickly, particularly with diarrhoea, vomiting or fever. Signs can include fewer wet nappies, few or no tears, sunken eyes, unusual drowsiness or a sunken soft spot on a baby’s head.
The NHS advises continuing breastfeeding or formula feeding, offering smaller amounts more often if needed, and asking a pharmacist about suitable oral rehydration solutions. Do not dilute baby formula to make it weaker. The right approach to additional water depends on a baby’s age and feeding, so follow NHS or professional advice.
Older adults may be more vulnerable, especially if they have difficulty obtaining drinks or rely on someone else to help. Offer drinks regularly and make them accessible, while respecting any medical fluid restrictions.
If a child under five has signs of dehydration, contact NHS 111 or seek urgent clinical advice. The NHS also says to call 111 if you need advice for a child under five rather than relying on the online 111 service.
When is dehydration serious?
Most mild dehydration can improve with suitable fluids, but some symptoms need medical attention. The following is based on the NHS dehydration advice.
| Level of concern | Signs | Action |
|---|---|---|
| Possible mild dehydration | Thirst, dry mouth, darker urine | Take appropriate fluids and monitor how you feel |
| Urgent medical advice | Persistent dizziness on standing, unusual drowsiness, reduced urination, fast heartbeat or breathing, or concerning signs in a child | Urgent GP appointment or NHS 111 |
| Emergency | Signs of dehydration with confusion, difficulty breathing, cold skin, blue/grey/pale or blotchy skin, or difficulty waking | Call 999 or go to A&E; do not drive yourself |
Do not delay seeking help if someone is severely unwell. The NHS notes that the most serious symptoms can indicate shock.
Eight simple ways to stay better hydrated
- Have a drink with meals and at regular points during the day.
- Keep a refillable bottle or glass nearby when working or studying.
- Increase suitable fluid intake when sweating more than usual.
- Remember that illness involving fever, vomiting or diarrhoea may increase fluid needs.
- Choose water or other lower-sugar drinks for everyday hydration.
- Use thirst and pale-yellow urine as everyday clues, without treating them as perfect medical tests.
- Offer drinks to children or older people who need help remembering or getting them.
- Follow professional advice if you have a medical condition requiring fluid management.
Small, repeatable habits are generally more useful than trying to make up for an entire day of missed drinks in one sitting.
Frequently asked questions
Is dark urine always a sign of dehydration?
No. Concentrated dark yellow urine can suggest dehydration, but foods, vitamins, medicines and illness can also change urine colour. Persistent or worrying changes deserve medical advice.
Can dehydration make you sleepy?
Yes, tiredness can occur. However, unusual drowsiness—particularly with other symptoms—can be a sign of serious dehydration and needs urgent attention.
Do I have to drink eight glasses of plain water?
No. The NHS guideline is six to eight cups or glasses of fluid, and suitable drinks other than plain water also count. Individual requirements vary.
Should I drink extra water when I am ill?
Illness can increase fluid losses, especially with fever, vomiting or diarrhoea. Small sips may be easier if you feel sick, and a pharmacist can advise whether oral rehydration solution is appropriate.
Can babies drink extra water?
Advice depends on age and whether the baby is breastfed, formula-fed or on solids. Follow the NHS guidance and seek prompt help for signs of dehydration. Never weaken formula.
Does drinking more water cure headaches?
Only when inadequate hydration is contributing might it help. Headaches have many causes, and sudden or severe symptoms may need medical assessment.
The bottom line
When you do not drink enough to replace the fluid you lose, your body starts conserving water. Thirst, dry mouth, concentrated urine, headache, dizziness and tiredness may follow. Vomiting, diarrhoea, hot weather and certain medicines can increase the risk.
For most people, regular drinks and awareness of fluid losses are practical starting points. But dehydration with persistent dizziness, concerning symptoms in children or signs of severe illness needs prompt medical assessment.
This is general health information, not a diagnosis or a substitute for personalised medical advice. Sources: NHS dehydration and NHS water, drinks and hydration.