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Health condition · Clinically reviewed

Dehydration, from a glass of ORS to IV fluids in resus.

More than thirst - a mismatch between water lost and water taken in that can quietly slide into shock in the very young, the very old and anyone with an ongoing loss.

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Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against NICE, BPS and peer-reviewed sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK guidance on oral rehydration, IV fluid resuscitation and safe sodium correction.

Key facts

Dehydration at a glance.

The essentials, in plain English - what it is, the three types, who is most at risk, and how it is treated in the UK today.

  • What it is

    Loss of body water greater than intake, producing fluid and electrolyte imbalance across cells and circulation.

  • Three types

    Isotonic (equal water and salt loss), hypertonic (water loss dominant, high sodium) and hypotonic (salt loss dominant, low sodium).

  • Common triggers

    Gastroenteritis, fever, heat, exercise, diuretics, uncontrolled diabetes and reduced intake in older adults.

  • Who is vulnerable

    Older adults, young children, athletes, people in hot climates and anyone with chronic illness or reduced mobility.

  • First-line therapy

    Oral rehydration solution (Dioralyte or WHO ORS) plus regular sips is enough for most mild and moderate cases.

  • When it is urgent

    Reduced consciousness, shock, severe electrolyte disturbance, acute kidney injury or heat stroke is an emergency.

Why this guide matters

Simple to treat early, dangerous when missed.

Dehydration is one of the most common and most under-recognised presentations in UK primary care and A&E. The three points below shape everything else on this page.

  • Cause first, fluids second

    Vomiting, diarrhoea, fever, uncontrolled diabetes and diuretics all need different follow-through - fluids alone are rarely enough.

  • Oral rehydration works for most

    Dioralyte or WHO ORS in small, frequent sips resolves most mild and moderate dehydration without an IV line.

  • Correct electrolytes slowly

    Sodium changes above 10 mmol per day risk brain injury - hospital replacement is monitored and deliberate.

How the diagnosis is made

From first sip to a clear fluid plan.

The steps a UK GP or hospital team will normally follow, in order - so you know what to expect and why.

  1. 01

    Assessing

    Clinical assessment and vitals

    A structured look at thirst, mucous membranes, skin turgor, capillary refill, pulse, blood pressure and postural drop.

  2. 02

    Assessing

    Fluid balance and weight

    Recent weight loss, urine output and a running fluid balance chart help estimate the deficit.

  3. 03

    Assessing

    Look for the cause

    Vomiting, diarrhoea, fever, heat exposure, diuretics, uncontrolled diabetes or reduced access to fluids all point somewhere different.

  4. 04

    Confirming

    Bloods and urine

    FBC, U&Es (sodium, urea, creatinine), glucose and urine osmolality confirm severity and type of dehydration.

  5. 05

    Confirming

    Paediatric scoring

    In children, WHO and BPS clinical dehydration scores guide the level of care and rehydration route.

  6. 06

    Planning

    Deficit calculation

    A percentage-of-body-weight deficit shapes the volume and speed of oral or IV rehydration.

  7. 07

    Planning

    Escalation and specialist input

    Severe cases, AKI, unresolved electrolyte disturbance or an underlying cause may need renal, endocrine, paediatric or ICU review.

Typical timeline: a first assessment to a settled fluid plan within a single visit.

Symptoms

What dehydration actually looks like.

Thirst and dark urine come first, then circulation and cognition. The features below are the ones clinicians look for at the bedside.

  • Thirst and dry mouth

    The earliest, most reliable early signs, though blunted in older adults and small children.

  • Reduced skin turgor

    Pinched skin on the forearm or sternum returns slowly - a bedside marker of interstitial fluid loss.

  • Dark, concentrated urine

    Low volume and dark colour with rising urine osmolality point to ongoing renal water conservation.

  • Postural dizziness

    Light-headedness on standing, with a drop in blood pressure, reflects reduced circulating volume.

  • Tachycardia and hypotension

    The circulatory response to volume loss, appearing before overt shock in most adults.

  • Oliguria

    Passing less urine than usual, often with pre-renal acute kidney injury on bloods.

  • Confusion and drowsiness

    Cognitive change in older adults may be the first and only clue - always check hydration and sodium.

  • Red flag - reduced consciousness or shock

    Collapse, cool peripheries, prolonged capillary refill or a very low blood pressure needs emergency care.

Treatment

How dehydration is treated in the UK.

Oral rehydration for most, IV fluids for the severely unwell, careful electrolyte correction throughout - and always treatment of the underlying cause.

  • Oral rehydration solution

    Dioralyte or WHO ORS in small, frequent sips - the first-line treatment for most mild and moderate dehydration.

  • Increased plain fluids

    Water, weak squash or broth alongside ORS - useful in mild losses without significant electrolyte disturbance.

  • IV normal saline

    Rapid isotonic replacement for severe dehydration, shock or when oral intake is not tolerated.

  • Hartmann’s solution

    A balanced crystalloid used for resuscitation and maintenance, especially where large volumes are needed.

  • Electrolyte correction

    Careful replacement of sodium, potassium and magnesium - sodium changes limited to under 10 mmol per day to avoid brain injury.

  • Antiemetics and selective antidiarrhoeals

    Used cautiously to reduce ongoing losses when vomiting or diarrhoea is preventing oral rehydration.

  • Treating the underlying cause

    Antibiotics for infection, insulin for DKA, desmopressin for cranial DI, dose review of diuretics and treatment of hyperthyroidism.

  • Cooling for heat illness

    Move to a cool environment, cold IV fluids, evaporative cooling or cold-water immersion for heat stroke - always with hospital care.

What this guide is based on

The sources behind every claim on this page.

UK national guidance and specialist society standards, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP or hospital team knows your history and can tell you which parts apply to you. If you or someone you care for is unwell, get seen.

  • NICE. Intravenous fluid therapy in adults in hospital (CG174).

  • NICE. Diarrhoea and vomiting in children under 5 (CG84).

  • British Paediatric Society and RCPCH. Paediatric fluid and dehydration guidance.

  • World Health Organization. Oral rehydration salts (ORS) formulation and use.

Red flags

When dehydration needs urgent care.

Most dehydration is managed at home or in primary care. These are the situations where hospital assessment cannot wait.

  • Reduced consciousness

    Drowsiness, confusion progressing to unresponsiveness needs an emergency assessment and usually IV fluids.

  • Shock

    Cool peripheries, prolonged capillary refill, tachycardia and low blood pressure require immediate resuscitation.

  • Severe electrolyte disturbance

    Very high or very low sodium, potassium or calcium can cause seizures and arrhythmias - correct in hospital.

  • Acute kidney injury

    Rising creatinine and falling urine output alongside dehydration is common and needs careful fluid and drug review.

  • Heat stroke

    Core temperature above 40C with neurological features is a medical emergency needing rapid cooling and hospital care.

  • Child with severe dehydration

    Sunken eyes, lethargy, prolonged capillary refill or shock in a child needs urgent paediatric assessment.

  • Uncontrolled underlying disease

    New diagnosis of diabetes, diabetes insipidus or an addisonian crisis can present as dehydration and demands specialist input.

  • Rapid sodium correction risk

    Correcting sodium too quickly risks central pontine myelinolysis or cerebral oedema - keep changes under 10 mmol per day.

  • Frail older adult at home

    Reduced intake, new confusion or a fall in a dependent adult should prompt an urgent GP or same-day service review.

Prevention

A preventable problem, with a clear routine.

Four things that make the biggest difference day to day - regular sips, early ORS, help for older adults, and knowing when to escalate.

A quiet reminder

Little and often beats a big glass at bedtime.

Steady sips through the day keep circulation and kidneys happy, especially in heat, illness or after exercise.

  1. 01 Prevention

    Drink to a routine

    Regular sips through the day beat large drinks now and then, especially in hot weather, illness or during exercise.

  2. 02 Illness

    Reach for ORS early

    Start Dioralyte or WHO ORS at the first sign of vomiting or diarrhoea rather than waiting for symptoms to worsen.

  3. 03 Carers

    Help older adults

    Prompt fluids at set times, keep a drink in reach and watch for confusion, dark urine or reduced output.

  4. 04 Escalate

    Know when to get seen

    Drowsiness, dizziness on standing, very little urine, a very unwell child or a frail adult all warrant urgent review.

Frequently asked

Everything we get asked about dehydration.

Quick answers on causes, oral rehydration, IV fluids and safe sodium correction.

  • What is dehydration?

    Dehydration is a state where the body loses more water than it takes in, producing fluid and electrolyte imbalance. It can be isotonic (equal water and salt loss), hypertonic (water loss dominant, high sodium) or hypotonic (salt loss dominant, low sodium), and it ranges from mild thirst to shock and coma.

  • What are the common causes?

    Reduced intake in older adults or people with neurological illness, and increased losses from vomiting and diarrhoea, uncontrolled diabetes, diuretics, fever, hot weather, heavy exercise, burns or major inflammation such as pancreatitis and sepsis.

  • How do I know if it is serious?

    Warning features include reduced urine output, dark urine, dizziness on standing, a fast pulse, low blood pressure, drowsiness or confusion. In children, sunken eyes, lethargy and prolonged capillary refill are red flags. Any of these needs urgent medical review.

  • When is oral rehydration enough?

    For most adults and children with mild or moderate dehydration who can drink and hold fluids down, oral rehydration solution (Dioralyte or WHO ORS) in small, frequent sips is usually enough alongside treatment of the underlying cause.

  • When are IV fluids needed?

    IV fluids are used for severe dehydration, shock, persistent vomiting, reduced consciousness, significant electrolyte disturbance or acute kidney injury. Normal saline or Hartmann’s solution is used, with careful monitoring and gradual electrolyte correction.

  • Why does sodium have to be corrected slowly?

    Rapid correction of a low sodium can cause central pontine myelinolysis, and rapid correction of a high sodium can cause cerebral oedema. Guidance is to change sodium by no more than about 10 mmol per litre in 24 hours, in hospital, with regular blood tests.

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