Dehydration vs Flu
By the AllHealth UK editorial teamLast reviewed Next review due Compiled from published NHS and NICE guidance. Not reviewed by a clinician.
Am I dehydrated or coming down with something?
Dehydration and a viral illness both cause headache, tiredness and feeling generally awful, and one often causes the other. Dehydration shows in your pee: dark yellow, strong-smelling and less often than usual, with a dry mouth and no high temperature. Flu brings a sudden high temperature, aching muscles and chills. Dehydration usually improves within hours of drinking enough.
Immediate action required: call 999 or go to A&E if
- Blue, grey, pale or blotchy skin, lips or tongue — on brown or black skin this may be easiest to see on the palms of the hands or soles of the feet
- Skin that feels cold
- Difficulty breathing, or taking lots of quick breaths
- Confusion, or being sleepier than normal or difficult to wake
- Sudden chest pain
- Severe difficulty breathing — gasping, choking, or unable to get words out
- Coughing up blood
If you are weighing up whether it is bad enough to call, that is itself a reason to call. Nobody is annoyed by a 999 call that turns out to be the less serious of the two.
| Feature | Dehydration | Flu |
|---|---|---|
| Typical signs |
|
|
| How long it lasts | Improves within hours of drinking enough fluid | 1–2 weeks, with tiredness lingering longer |
| What helps | Drink fluids, starting with small sips if you feel sick. Avoid alcohol and caffeine. A pharmacist can recommend an oral rehydration solution if you have been sick or had diarrhoea. Call 111 or ask for an urgent GP appointment if you have dark yellow pee or are peeing less than normal, feel unusually tired or drowsy, or feel dizzy on standing and it does not go away — these can be signs of serious dehydration. | Rest, plenty of fluids, paracetamol. Antivirals may be offered if you are in an at-risk group. Call 111 if you are 65 or over, pregnant or recently gave birth, have a long-term condition or weakened immune system, feel very unwell or short of breath, or are no better after 7 days. |
| What to do | Self-care at home | Self-care at home |
Sources and further reading
This page was written from the following published guidance. All links checked 26 August 2026.
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