Dehydration happens when the body loses more water than it takes in. Mild dehydration is common, easy to fix and usually harmless. Moderate or severe dehydration is a different matter, especially in babies, older adults and people who are already unwell. The difficulty is that the early signs are vague, and the point at which “drink some water” stops being enough is not always obvious.
This guide covers how to recognise dehydration, what to drink, who is most at risk, the warning signs that need medical help, and the situations where drinking more is actually the wrong advice. If you are weighing up sachets and sports drinks, our guide to electrolyte drinks covers when they earn their place.
- Why dehydration happens
- Recognising the signs
- Why older adults are more at risk
- Treating mild dehydration at home
- When drinking more is the wrong advice
- When dehydration becomes heat illness
- Warning signs that need medical help
- Preventing dehydration day to day
- Medicines that need extra care when you are dehydrated
- Questions people ask
- Sources
Why dehydration happens
Water leaves the body constantly through urine, sweat, breath and stool. Most days, thirst and ordinary eating and drinking keep the balance. Dehydration develops when losses rise or intake falls, and often both at once. Common causes include vomiting and diarrhoea, which remove salts as well as water; fever, which increases water loss through the skin and breath; heat and heavy sweating during work or exercise; high blood sugar, which draws water out through the urine; medicines such as diuretics; and simply not drinking enough, because of illness, busyness, difficulty swallowing or worry about toilet trips.
As fluid is lost, blood volume falls. The heart beats faster to compensate, the kidneys conserve water, and blood pressure can drop, particularly on standing. That is why dizziness and a racing pulse are signs to take seriously.
Recognising the signs
| Level | Common signs | What to do |
|---|---|---|
| Mild | Thirst, dry mouth, darker urine, headache, tiredness | Drink steadily, rest, cool down |
| Moderate | Dizziness on standing, very little urine, very dry mouth, weakness, faster heartbeat | Seek medical advice the same day; sip fluids if able |
| Severe | Confusion, fainting, no urine for many hours, rapid breathing, extreme weakness, difficulty waking | Emergency care |
The NHS guide to dehydration lists feeling thirsty, dark yellow and strong-smelling urine, dizziness, tiredness, a dry mouth and peeing fewer than four times a day among the common signs. Mayo Clinic adds that thirst is not always a reliable early signal, particularly in older adults.
In babies and young children, look for fewer wet nappies, no tears when crying, a sunken soft spot on the head, unusual sleepiness or irritability. Children can become dehydrated quickly with vomiting and diarrhoea and should be assessed early.
Urine colour: a clue, not a verdict
Pale straw-coloured urine usually means you are drinking enough, and dark amber urine often means you need more. But B vitamins, some medicines and certain foods change the colour independently. Very clear urine all day can simply mean you are drinking more than you need. Use colour alongside how often you are going and how you feel.
Why older adults are more at risk
Several changes stack up with age. Thirst becomes weaker, so the body’s own alarm rings later. Total body water falls as muscle mass declines. Kidneys become less efficient at conserving water. Many older adults take medicines that affect fluid balance, and some deliberately drink less to avoid night-time toilet trips or accidents. Memory problems, reduced mobility and swallowing difficulties add further barriers.
The consequences can be serious. Dehydration in an older person can show up as sudden confusion, weakness or dizziness rather than obvious thirst, and it raises the risk of falls, urinary infections, constipation and kidney problems. Heat waves are a particular danger; the National Institute on Aging’s hot weather advice explains how to stay safe. If falls are a concern, our guide to fall prevention covers the wider picture.
If you are caring for someone
Link drinks to routines, such as one with each meal, each medicine round and each visit, and keep a preferred drink within easy reach, in a cup that is easy to hold. Offer fluid-rich foods such as soup, yoghurt, jelly and fruit, and make the route to the toilet safe and easy, especially at night. If drinking causes coughing or choking, follow the person’s swallowing plan and ask for a review rather than pushing larger drinks.
Treating mild dehydration at home
Home care is appropriate for mild symptoms in someone who is alert and able to drink safely.
- Sip steadily. Small, frequent drinks are better tolerated than large ones, especially with nausea.
- Use an oral rehydration solution when salts are being lost. With vomiting, diarrhoea or heavy sweating, a pharmacy rehydration sachet made up exactly as directed replaces water, sodium, potassium and glucose in a balance designed for absorption. Sports drinks and fruit juice are not formulated for this and can contain too much sugar.
- Cool down and rest. Stop exercise, move to shade or a cooler room, and loosen clothing.
- Keep eating if you can. Soup, fruit and ordinary meals add fluid and salts.
- Avoid alcohol until you feel normal again.
For babies, continue breast or formula feeds and seek advice from a health professional early. Do not dilute formula or use juice as a rehydration treatment. Cleveland Clinic’s overview covers the differences in treatment between age groups.
When drinking more is the wrong advice
General advice to “drink plenty” does not apply to everyone. People with heart failure, advanced kidney disease or liver disease are sometimes given a fluid limit, because extra water can cause swelling, breathlessness or dangerously low sodium. If you have been given a fluid restriction, ask your clinician for a personal sick-day plan that tells you what to do if you have vomiting, diarrhoea or a fever.
Very large volumes of plain water in a short time can also lower blood sodium in healthy people, a risk seen occasionally in endurance events. And if you take diuretics, blood pressure medicines or diabetes medicines, do not change the doses yourself during an illness; ask a pharmacist or clinician. People with diabetes should follow their sick-day rules, which usually include more frequent glucose checks.
When dehydration becomes heat illness
In hot weather, dehydration and overheating feed each other. Less fluid means less sweat, and less sweat means the body cools less effectively. The result can move from simple dehydration to heat exhaustion, and then to heatstroke, which is a medical emergency.
| Heat exhaustion | Heatstroke | |
|---|---|---|
| Skin | Pale, cool, clammy, heavy sweating | Hot, may be red and dry or still sweaty |
| Mind | Tired, weak, possibly dizzy | Confused, agitated, slurred speech, collapse or seizure |
| Other signs | Headache, nausea, muscle cramps, fast pulse | Very high body temperature, rapid breathing |
| What to do | Move somewhere cool, lie down, loosen clothing, sip fluids; seek help if not better within about 30 minutes | Call emergency services, cool the person rapidly while waiting, do not give drinks if they are not fully alert |
Older adults, young children, outdoor workers and people with heart disease are most vulnerable, as are people taking medicines that affect sweating or fluid balance. On very hot days, plan activity for cooler hours, check on older neighbours and relatives, and keep drinks within reach. A cool shower, a damp cloth on the skin and a fan can help lower body temperature at home, though fans alone are less effective in extreme heat.
Warning signs that need medical help
Call emergency services if someone is confused, has fainted or had a seizure, is very hard to wake, is breathing rapidly, or cannot drink safely.
Seek same-day medical advice if:
- vomiting means fluids cannot be kept down
- there has been little or no urine for eight hours or more
- diarrhoea is severe, bloody or lasting more than a couple of days
- symptoms are not improving despite drinking
- an older adult suddenly becomes weak, dizzy or muddled
- a baby or young child shows any of the signs above
Preventing dehydration day to day
There is no single number of glasses that suits everyone. Needs vary with body size, activity, climate, diet and health. The practical goal is to drink regularly through the day, more in heat and during illness, rather than waiting to feel parched. A few habits help: have a drink with every meal and keep water nearby, and plan ahead for hot days, long journeys and exercise. Tea and coffee count too, since for most people at usual amounts they contribute to fluid intake, though alcohol does not, as it increases urine output. Eating water-rich foods such as fruit, vegetables, soups and yoghurt helps, and during any illness with fever, vomiting or diarrhoea, it is worth starting extra fluids early.
Dry skin is not the same as a dehydrated body; our article on dry skin versus dehydrated skin explains the difference.
Medicines that need extra care when you are dehydrated
Some common medicines either increase fluid loss or become riskier when the body is short of fluid, particularly during vomiting, diarrhoea or a fever. Many health services give people on these medicines “sick day rules”: advice to pause certain tablets temporarily while they are unable to eat and drink normally, and to restart them once they have recovered. The exact advice depends on your condition, so it should come from your own doctor or pharmacist.
| Medicine group | Why dehydration matters |
|---|---|
| Diuretics (“water tablets”) | They increase fluid and salt loss, which adds to losses from illness or heat |
| ACE inhibitors and angiotensin receptor blockers (for blood pressure and heart failure) | Combined with dehydration, they can reduce kidney blood flow |
| Metformin | Kidney function can drop during dehydration, which affects how the medicine is cleared |
| SGLT2 inhibitors (some diabetes and heart medicines) | They increase fluid loss through urine and carry a rare risk of ketoacidosis when unwell |
| Anti-inflammatory painkillers such as ibuprofen | They can strain the kidneys when fluid is low |
| Lithium | Dehydration can push blood levels up to a harmful range |
| Laxatives | Overuse causes fluid and salt loss |
If you take any of these and develop a stomach bug or cannot keep fluids down for more than a day, contact your doctor, pharmacist or a health advice line rather than waiting it out.
Questions people ask
Can you be dehydrated without feeling thirsty?
Yes, particularly in older adults, whose thirst signal is weaker. That is why routines matter more than waiting for the urge to drink.
Does coffee dehydrate you?
Not at usual intakes for most people. The fluid in the cup outweighs the mild effect on urine output.
Can I make my own rehydration drink?
Homemade mixtures are easy to get wrong, and too much salt or sugar can make things worse. Use a pharmacy product where possible, and follow local health guidance if one is not available.
Sources
- NHS: Dehydration
- Mayo Clinic: Dehydration, symptoms and causes
- Cleveland Clinic: Dehydration
- National Institute on Aging: Hot weather safety for older adults
Sources checked 15 September 2026. This article is general education, not a diagnosis. See our editorial policy and medical disclaimer.
