Signs of Dehydration: The Headache, the Pee Test and How to Rehydrate

"Dehydration means your body loses more fluids than you take in," in the NHS's words, and the signs in adults are thirst, a headache, dark yellow strong-smelling pee, peeing less often, dizziness, tiredness and a dry mouth. Pale pee is the everyday check, drinking is the fix, a pharmacist's rehydration solution is for vomiting and diarrhoea, and the NHS sets the 111 and 999 lines.¹ It is part of the Wellgard electrolyte guide.

Evidence at a glance

• Trials: three drinking trials (102, 72 and 12 people), a Cochrane review of 24 datasets in over-65s, two urine-colour studies, one 34-person headache series.
• Largest single trial: 102 adults with frequent headaches, 1.5 litres more water a day for 3 months.²
• Strongest measured outcome: over 4 hours, 72 men passed about 1.04 kg of urine after a litre of oral rehydration solution and 1.05 kg after milk, against 1.34 kg after water, in a trial funded by a hydration research institute whose advisory board the lead author chaired.³
• Not tested: no trial has given an electrolyte drink or sachet for a dehydration headache.

Sources checked on 13 October 2026.

What are the signs of dehydration in adults?

The NHS list is feeling thirsty, a headache and feeling light-headed, dark yellow strong-smelling pee, peeing less often than usual, feeling dizzy, feeling tired, a dry mouth, lips and tongue, and sunken eyes.¹ Women and men share the list; no source gives a sex-specific sign.

The NHS adds that "babies, children and older adults are more at risk of dehydration." It comes more easily with diabetes, sickness or diarrhoea, heat, alcohol, heavy sweating, a high temperature, medicines that make you pee more (diuretics), or relying on others for drinks.¹ So you can drink plenty and still be dehydrated when losses run faster. Hangovers have their own page, what to eat and drink with a hangover, and if tirzepatide is behind sickness or diarrhoea, dehydration on tirzepatide covers what its leaflet says. Babies show a sunken soft spot, few tears and fewer wet nappies.¹

Can you tell if you are dehydrated from your pee?

Roughly, yes, for a healthy adult. The NHS check is to drink enough through the day that your pee is "a pale colour"; pale, not colourless, is the target.¹ Colour is a field guide, not a lab test: close enough for sport and work settings, not where precision matters.⁴

In the stronger study, nine trained men were dehydrated to 3.7% of body weight, cycled to exhaustion and rehydrated within 42 hours, and colour tracked their body water as well as, or better than, urine and blood tests including plasma sodium.⁵ Beetroot can turn pee pink and some medicines turn it red or brown; pink, red or dark brown pee without such a reason may be blood and needs an urgent GP appointment or 111.⁶ Over 65, colour is not reliable (below).

What does a dehydration headache feel like, and how long does it last?

Usually a dull ache that worsens when you move your head, bend down or walk. In a 2004 London case series of 34 people, 22 had complete relief within 30 minutes of drinking 200 to 1,500 ml of water (500 ml on average) and 11 within one to three hours of 500 to 1,000 ml (750 ml on average).⁷

It is a small interview study with no blood tests; about 1 in 10 of those asked recognised the headache.⁷ In a later survey, 34 of 95 people with migraine named going without fluid as a trigger.⁸ The NHS lists low fluid intake as a common cause, but also lists a headache "made worse by coughing, sneezing, bending down or exercising" as a reason for an urgent GP appointment or 111, so if one has not lifted a few hours after drinking, make that call. A headache that "started suddenly and is extremely painful" is a 999 call.⁹

For frequent headaches there is one randomised trial: of 102 primary-care patients drinking under 2.5 litres a day, 52 were told to add 1.5 litres of water. After three months they scored 4.5 points higher on a migraine quality-of-life scale and 47% against 25% reported much improvement, but headache days did not change in any relevant way.²

How do you rehydrate, and how fast?

Drink. If you feel sick or have been sick, the NHS says to "start with small sips and then gradually drink more", and to avoid caffeinated drinks and alcohol while treating dehydration.¹ If vomiting or diarrhoea is taking too much fluid, a pharmacist can recommend an oral rehydration solution, a powder that puts back lost sugar and salts. For everyday prevention, water or diluted sugar-free squash are the NHS choices.¹

In a 2016 UK trial funded by a hydration research institute whose advisory board the lead author chaired, 72 healthy men who were not dehydrated drank a litre of water or one of 12 other drinks; less urine followed an oral rehydration solution or milk than water over 4 hours, while tea, coffee, lager, juice and a sports drink were no different from water.³ In 12 men dehydrated by about 2% of body weight through cycling, a 61 mmol/L sodium drink left less urine than a 23 mmol/L one at the same volumes; drinking more than you lost restored balance only with enough sodium.¹⁰

So water is right for most days. What the 1996 and 2016 trials measured was retention: the 61 mmol/L drink left less urine than the 23 mmol/L one, the oral rehydration solution left less than water, and no sugar-free electrolyte sachet was tested.^{3, 10} With a stomach bug the NHS route is a pharmacist's oral rehydration solution; after hours of sweat, sodium in the drink or the meal is where that evidence sits. Illness has its own rules, set out under rehydration sachets when you are ill. No source gives a recovery time; dizziness on standing that does not go away is a 111 matter.¹ The opposite problem: dehydration vs hyponatraemia in long events.

Why are older people different?

Because the usual checks fail. A 2015 Cochrane review of 24 datasets in people aged 65 and over found that fluid intake, urine colour, urine volume, heart rate, dry mouth and feeling thirsty "are not useful, and should not be relied on individually".¹¹

The one promising pair came from a single study of 71 people: missing drinks between meals plus saying they felt tired picked out about 7 in 10 of those dehydrated and wrongly flagged fewer than 1 in 10 of the rest (sensitivity 0.71, specificity 0.92).¹¹ For someone you care for, the NHS suggests drinks at mealtimes, watery foods such as soup, jelly and melon, and drinks always within reach. Confusion, or being sleepier than normal, alongside signs of dehydration is a 999 matter.¹

When is dehydration a 111 or 999 matter?

Ask for an urgent GP appointment or contact NHS 111 if you feel unusually tired, feel dizzy on standing and it does not go away, have dark yellow pee or are peeing less than normal, or are breathing quickly or have a fast heart rate: possible "signs of serious dehydration that need urgent treatment". Call 111 for advice about a child under 5.¹

Call 999 or go to A&E if someone with signs of dehydration has blue, grey, pale or blotchy skin, lips or tongue (on brown or black skin, easier to see on the palms or soles), skin that feels cold, difficulty breathing, confusion, or is sleepier than normal or difficult to wake. The NHS says these "could be signs of shock".¹

What we don't know

• No trial has given an electrolyte drink or sachet for a dehydration headache, which rests on one 34-person interview series.
• The 13-drink trial enrolled only healthy men who were not dehydrated, so women, older adults and people who were ill were not measured.
• No single home check has proved reliable in people over 65; the one promising pair comes from a single 71-person study.

Frequently asked questions

Are dehydration and low electrolytes the same thing?

No. Dehydration is a fluid deficit; a low blood sodium, potassium or magnesium is a separate problem found by a blood test. They overlap after vomiting or diarrhoea.¹ The signs of low electrolytes covers the other side.

How do you test for dehydration at home?

In a healthy adult, thirst and pee colour are the practical checks, with the limits above; over 65, no single home sign is reliable.^{1, 5, 11} No home device reads blood sodium; that takes a blood test arranged by your GP.

Do electrolytes help with dehydration?

The trials measured retention, not a product: in the 2016 trial an oral rehydration solution was kept longer than water, in the 1996 trial the drink with more sodium left less urine, and no sugar-free electrolyte sachet was tested.^{3, 10} With vomiting or diarrhoea, the NHS route is a pharmacist's oral rehydration solution.¹

Can electrolytes cause headaches?

No trial has tested whether electrolyte drinks cause or relieve headaches. A headache that started suddenly and is extremely painful is a 999 call.⁹

What is the best drink for dehydration in adults?

Day to day, water or diluted sugar-free squash; with vomiting or diarrhoea, a pharmacist's oral rehydration solution.¹ After hours of sweating, milk or a drink with sodium: the 2016 trial found milk was kept about as well as a rehydration solution, and the 1996 trial found the saltier drink left less urine.^{3, 10} Do electrolytes hydrate you better than water compares them.

Sources

1. NHS. Dehydration. Page last reviewed 1 May 2026; read 13 October 2026. nhs.uk/conditions/dehydration.
2. Spigt M, et al. Randomised trial, 102 adults with recurrent headaches, 3 months. Family Practice, 2012. PubMed 22113647.
3. Maughan RJ, et al. Randomised trial, 72 men, 13 drinks, 4 hours; funded by a hydration research institute whose advisory board the lead author chaired. American Journal of Clinical Nutrition, 2016. PubMed 26702122.
4. Armstrong LE, et al. Two laboratory studies and one field study; sizes not in the abstract. International Journal of Sport Nutrition, 1994. PubMed 7987361.
5. Armstrong LE, et al. Laboratory study, 9 trained men, 42 hours; funding not checked. International Journal of Sport Nutrition, 1998. PubMed 9841955.
6. NHS. Blood in urine. Page last reviewed 25 September 2026; read 13 October 2026. nhs.uk/conditions/blood-in-urine.
7. Blau JN, Kell CA, Sperling JM. Case series by interview, 34 people; funding not checked. Headache, 2004. PubMed 14979888.
8. Blau JN. Survey, 95 people with migraine. Headache, 2005. PubMed 15953311.
9. NHS. Headaches. Page last reviewed 17 April 2024; read 13 October 2026. nhs.uk/conditions/headaches.
10. Shirreffs SM, et al. Controlled trial, 12 men, 4 drink volumes; funding not checked. Medicine and Science in Sports and Exercise, 1996. PubMed 8897383.
11. Hooper L, et al. Cochrane diagnostic review, 24 datasets, people aged 65 and over. Cochrane Database of Systematic Reviews, 2015. PubMed 25924806.

General information, not medical advice. If you take a diuretic or have kidney or heart disease, ask your GP or pharmacist before using any rehydration or electrolyte product.

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