Dehydration means your body loses more fluid than it takes in. Mild signs are thirst, a dry mouth, darker urine, tiredness and headache; confusion, fainting, a very fast heartbeat or passing no urine for many hours are warning signs that need urgent medical help.
The most common signs of dehydration
Mild to moderate
- thirst,
- a dry mouth, lips and eyes,
- dark yellow, strong-smelling urine and passing urine less often,
- tiredness and weakness,
- headache or dizziness,
- muscle cramps.
Serious — seek medical help
- confusion, marked drowsiness or trouble concentrating,
- fainting, or dizziness on standing that does not go away,
- a very fast heartbeat or fast breathing,
- passing no urine for many hours,
- seizures,
- in babies and young children: a sunken soft spot on the head, no tears, few wet nappies, unusual drowsiness.
Causes and risk factors
Dehydration usually has one or more of these causes:
- drinking too little, for example in a busy day,
- hot or humid weather and intense exercise with heavy sweating,
- vomiting, diarrhoea and fever,
- some conditions (such as diabetes) and medicines (such as diuretics),
- alcohol.
Babies and young children, older adults (who feel thirst less), people with long-term illness, athletes and outdoor workers are particularly at risk.
When to see a doctor
With serious signs (confusion, fainting, a very fast heartbeat, no urine, seizures) do not wait — get urgent help through your local emergency number. See a doctor also if vomiting or diarrhoea continues and you cannot keep fluids down, and early for babies, young children and older people, who dehydrate faster.
With diarrhoea and vomiting, oral rehydration solutions available at a pharmacy can help — ask a pharmacist about a suitable product.
Limits of thirst and urine colour
Thirst appears once a fluid deficit already exists and can be weaker in older adults. Urine colour is a handy clue, but it is also changed by B vitamins, medicines and some foods (such as beetroot). Very pale urine after drinking a large amount at once does not necessarily mean optimal hydration. Judge by several signals together — and talk to a doctor if in doubt.
How to prevent dehydration
- drink regularly through the day, not only when you feel very thirsty,
- drink more in hot weather, during exercise and in illness with fluid loss,
- eat fruit and vegetables — they also supply water,
- with vomiting or diarrhoea, drink small amounts often,
- avoid exertion in the hottest hours.
The WODA app helps you build a habit of regular drinking (reminders, logging the water you drink), but it does not diagnose dehydration and does not replace a doctor. How much to drink day to day: How Much Water Should You Drink Per Day?.
Frequently asked questions
How quickly do symptoms of dehydration appear?
It depends on conditions. In heat and during intense exercise the first signs can appear within hours, and with vomiting or diarrhoea even faster.
Can a headache be a sign of dehydration?
It can, but headaches have many other causes. If it is severe, sudden or comes with other worrying symptoms, see a doctor.
Is plain water enough for dehydration?
In mild cases, yes. With diarrhoea and vomiting you also lose electrolytes, so oral rehydration solutions help; severe dehydration needs medical treatment.
How can I tell if a child is dehydrated?
Warning signs include no tears, a dry mouth, a sunken soft spot in babies, passing urine rarely and unusual drowsiness. Small children dehydrate quickly — contact a doctor if in doubt.
Sources
- NHS — Dehydration
- Mayo Clinic — Dehydration: symptoms and causes
- EFSA — Scientific Opinion on Dietary Reference Values for water (EFSA Journal 2010;8(3):1459)
This article is for information only and does not replace advice from a doctor or dietitian. If you are unsure or have worrying symptoms, contact a doctor.