Signs of Dehydration: What to Look For and What to Do
- By Benjamin Dickman
- Updated
- 10 min read
Thirst, a dry mouth and peeing less often than usual can all be signs of dehydration. Think about what’s happened that day, such as sweating in the heat or being unwell. A darker shade of urine is one clue among several. Here’s what to look for and when to get help.
What to do now
| How you feel | What to do |
|---|---|
| You feel thirsty or have a dry mouth and are otherwise well | Have a drink at a comfortable pace and rest if you need to. |
| You're unusually tired, pee less often, or feel dizzy when you stand and it doesn't go away | Ask for an urgent GP appointment or get urgent advice from NHS 111. |
| Someone is confused, has trouble breathing, is hard to wake, collapses or has a seizure | Get emergency medical help straight away. In the UK, call 999. |
A pharmacist can advise on rehydration after vomiting or diarrhoea. See the full NHS advice on dehydration.
Heat illness and overdrinking can cause some of the same symptoms. See the hot-weather guide and guide to drinking too much water for their warning signs.
On this page
Common signs of mild dehydration
Dehydration means your body is losing more fluid than you’re taking in. You might feel thirsty, tired, dizzy or have a headache or dry mouth. You might also pee less often or notice your urine looks darker and smells stronger. These symptoms can have other causes, so consider which ones you’re noticing and what else has happened recently.
Have you been exercising in the heat, been unwell with a fever, vomited or had diarrhoea? Alcohol and medicines that make you pee more can also increase the risk. Babies, young children and older adults are more vulnerable, especially if they need help getting drinks.
What urine colour can tell you
Pale urine during the day often goes with drinking enough. A darker yellow shade combined with peeing less than usual may suggest you need fluid. Your first wee of the day is often darker, and food, supplements, medicines and some health conditions can change the colour or smell.
Pay attention to changes from your usual pattern and how you feel. Aim for comfortable, regular drinks. Forcing very large amounts of water in a short time can be dangerous.
Headache, tiredness and dizziness need context
A headache or low energy after a hot commute may occur alongside dehydration, but those symptoms are not specific. Lack of sleep, missed meals, infection, migraine, medicines and many other problems can feel similar. Try a normal, comfortable drink and review the wider situation instead of assuming that more water is always the answer.
Dizziness that persists when you stand, unusual drowsiness, rapid breathing or a fast heart rate can signal more serious dehydration and warrant urgent advice. Do not drive or continue exercising if you feel faint, confused or unsafe. In the UK, NHS 111 can advise when symptoms are urgent but not immediately life-threatening.
How to respond to ordinary fluid loss
If you’re otherwise well and can drink normally, start with a comfortable amount of water and keep drinking gradually. If you feel sick, start with small sips and build up as you can. Food can also help once you feel ready to eat.
Pause strenuous activity and move somewhere cooler if you’ve been in the heat. Get medical advice if you still feel unwell, keep losing fluids or can’t keep a drink down.
When oral rehydration solution is different from water
Vomiting and diarrhoea can cause losses of water, sugar, salts and minerals. A pharmacist may recommend an oral rehydration solution formulated to replace them. This is different from an everyday electrolyte drink or homemade mixture: the proportions matter, particularly for children and people with health conditions. Follow the product and professional instructions.
Plain water remains suitable for much ordinary daily hydration, while a balanced diet supplies electrolytes for most people. Do not assume that every headache needs a sports drink or that an electrolyte powder makes unlimited fluid safe. Ongoing illness, high-output stomas and prescribed fluid plans require individual clinical guidance.
Urgent warning signs
Get urgent advice if you feel dizzy when you stand and it doesn’t go away, pee much less than usual, feel unusually tired, breathe quickly or have a fast heart rate. Confusion, difficulty waking, cold or blotchy skin and difficulty breathing need emergency help.
If someone feels faint or weak in the heat, stop the activity and help them move somewhere cooler. Confusion, collapse, a seizure or a very high temperature can be an emergency. Get help straight away.
Preventing the same pattern tomorrow
Once you feel better, think about what you can change. Keep a drink handy during a long meeting, fill a bottle before a hot journey or plan where you’ll stop for water during exercise. If you usually forget, try having a drink with breakfast or during a break.
Speak to a healthcare professional if you keep feeling very thirsty or dizzy, pee much more than usual or notice an unexplained change in how much you drink. HydrateWise provides educational planning estimates, not medical advice. Kidney, heart and liver conditions, pregnancy, illness and some medicines can change safe fluid intake. Follow personal clinical guidance where it applies.