Fever
A raised body temperature, usually part of the immune response to infection.
Last revised
Key takeaways
- Commonly defined as: 38C or above
- In babies under 3 months: 38C or above needs urgent assessment
- Most useful single change: Keep fluids up; this matters more than lowering the temperature.
- Commonest mistake: Alternating paracetamol and ibuprofen routinely without advice.
- See a doctor if any of the signs under "When to get help" below apply to you.
Why it matters
It is a sign rather than a disease, and the number matters far less than how the person looks and behaves.
The numbers, in one place
| Measure | Figure |
|---|---|
| Commonly defined as | 38C or above |
| In babies under 3 months | 38C or above needs urgent assessment |
| Paracetamol and ibuprofen | for discomfort, not to normalise the number |
| Do not give aspirin | to anyone under 16 |
| Dehydration | the main practical risk in most fevers |
What helps
- Keep fluids up; this matters more than lowering the temperature
- Treat for comfort rather than to reach a target reading
- Dress lightly rather than wrapping up or sponging cold water
- Check on the person regularly, including overnight
What to avoid
- Alternating paracetamol and ibuprofen routinely without advice
- Aspirin in under-16s
- Judging severity by the number alone
Where to go next
- What People Get Wrong About Fever
- Starting Out With Fever
- Vitamin C: What the Evidence Actually Says
- Zinc: What the Evidence Actually Says
When to get help
Call 999 for a rash that does not fade under pressure, a stiff neck with light sensitivity, a seizure, difficulty breathing, or a child who is floppy, will not wake or is unusually drowsy. Any fever of 38C or above in a baby under three months needs urgent assessment. Seek urgent advice for fever in anyone immunosuppressed or having chemotherapy.
Where this comes from
How this page was put together
Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarised or generalised beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.