Fever in Adults: What Temperature Means and When to Worry

A fever, also called a high temperature, is usually a sign that your body is responding to an infection or inflammation. Many fevers are caused by common viral illnesses and settle within a few days with rest, fluids and simple symptom relief. But fever can sometimes be a sign of something more serious, especially if it is very high, getting worse, lasting longer than expected, or comes with warning symptoms such as confusion, breathing difficulty, chest pain, a stiff neck, rash or signs of dehydration.

It can be worrying to see a high number on a thermometer, but the temperature reading is only one part of the picture. How unwell you feel, how quickly symptoms are changing, your age, medical history, immune system and other symptoms all matter.

This guide explains what temperature counts as a fever in adults, common causes, what you can do at home, and when to contact a pharmacist, GP, NHS 111 or 999.

Important: Seek urgent medical help if you have a fever with confusion, severe drowsiness, difficulty breathing, chest pain, blue or blotchy skin, a stiff neck, a non-blanching rash, severe headache, persistent vomiting, signs of sepsis, or you feel seriously unwell.

What temperature counts as a fever in adults?

Normal body temperature is often described as around 37°C, but it varies from person to person and can change during the day. It can be slightly lower in the morning and slightly higher later in the day. Exercise, warm rooms, hot weather, alcohol, dehydration and hormonal changes can also affect temperature readings.

In adults, a fever is usually considered to be a temperature of around 37.8°C or above. Some sources use 38°C as a simple cut-off for a high temperature. Either way, the exact number is less important than the full pattern of symptoms.

For example, a temperature of 38°C with a runny nose and mild aches may be a simple viral illness. But a temperature of 38°C with confusion, breathing difficulty, a stiff neck or a rapidly worsening condition needs urgent attention.

Common fever ranges

These ranges are only a general guide:

Around 37°C: often normal, depending on the person and time of day.
37.8°C to 38.9°C: usually considered a fever or high temperature.
39°C or above: a higher fever that may need closer attention, especially if you feel very unwell.
40°C or above: a very high temperature. Seek medical advice, especially if it does not come down or comes with worrying symptoms.

A thermometer is useful, but you should not rely on it alone. Feeling faint, confused, breathless, dehydrated or unusually drowsy can be more important than the exact temperature.

What does a fever feel like?

Adults with a fever may feel hot, cold, shivery or sweaty. You may notice that your skin feels warm, your face looks flushed, or you cannot get comfortable.

Common symptoms that can come with a fever include:

shivering or chills
sweating
headache
muscle aches
tiredness or weakness
loss of appetite
thirst
faster heartbeat
feeling lightheaded
feeling generally unwell

A fever is not a diagnosis by itself. It is a clue that something is happening in the body. The next question is what else is happening alongside it.

Common causes of fever in adults

Most fevers in adults are caused by infections. These are often viral and improve on their own, but some bacterial infections need medical treatment.

Common causes include:

common cold and flu-like illnesses
COVID and other respiratory viruses
throat infections, including tonsillitis
chest infections
urinary tract infections
stomach bugs, norovirus or food poisoning
skin infections such as cellulitis
sinus infections
ear infections
inflammatory conditions
heat-related illness
some medicines or vaccine reactions
infections picked up during travel

Sometimes the cause is obvious from other symptoms. A fever with cough may point towards a respiratory infection. Fever with pain when urinating may suggest a urinary infection. Fever with vomiting and diarrhoea may suggest a stomach infection. Fever with a painful, swollen, red area of skin may suggest cellulitis or another skin infection.

You may find these related guides useful:

Fever with cough, sore throat or cold symptoms

A fever with cough, sore throat, runny nose, blocked nose, sneezing or body aches is often caused by a viral respiratory infection. This may include a cold, flu, COVID or another virus.

Most mild viral illnesses improve with rest, fluids and suitable pain relief. You may still feel tired for several days, and a cough can last longer than the fever.

Get medical advice if you have:

shortness of breath
chest pain
blue lips
confusion
symptoms that are getting worse rather than better
a fever that is not improving after a few days
a weakened immune system or significant long-term condition

If sore throat is a main symptom, see our guide to sore throat causes and when to see a GP. If cough is the main issue, read cough in adults.

Fever with diarrhoea or vomiting

A fever with diarrhoea, vomiting, stomach cramps or nausea may be caused by a stomach bug, food poisoning or another digestive infection. In many cases, symptoms improve within a few days.

The main risk is dehydration, especially if you cannot keep fluids down or are passing frequent watery stools.

Seek medical advice if you have:

blood in your stool
severe or worsening abdominal pain
persistent vomiting
signs of dehydration
fever after recent foreign travel
symptoms lasting longer than expected
a weakened immune system

Signs of dehydration include severe thirst, dark urine, peeing less than usual, dizziness, weakness, dry mouth and confusion. For more detail, see dehydration symptoms in adults and children.

Fever with pain when urinating

A fever with pain when urinating, needing to pee more often, lower abdominal pain or cloudy urine may suggest a urinary tract infection. If the infection reaches the kidneys, symptoms can become more serious.

Possible kidney infection warning signs include:

fever or chills
pain in the back or side
nausea or vomiting
feeling very unwell
confusion, especially in older adults

Seek medical advice promptly if you have urinary symptoms with fever, back pain, pregnancy, kidney problems, diabetes, a weakened immune system, or if you are male and have symptoms of a UTI.

Read more in our guide to pain when urinating.

Fever with rash

A fever with a rash can be caused by many different conditions, from viral illnesses to allergic reactions, scarlet fever, shingles, meningitis or sepsis. Some rashes are mild, but others need urgent care.

Get urgent help if a rash:

does not fade when pressed with a glass or finger
comes with a stiff neck, severe headache or light sensitivity
comes with confusion or extreme drowsiness
comes with breathing difficulty or swelling of the lips, tongue, throat or face
is rapidly spreading or associated with severe pain
appears with fever and you feel very unwell

If you are unsure whether a rash is serious, seek advice. Rashes can be difficult to judge without seeing them.

Fever after recent travel

A fever after recent foreign travel should be taken seriously, especially if you have visited an area where malaria, dengue, typhoid or other infections are possible.

Contact a GP or NHS 111 if you develop a fever after travel, particularly if you also have:

severe headache
rash
diarrhoea or vomiting
jaundice or yellowing of the skin or eyes
confusion
shortness of breath
severe muscle or joint pains

Tell the clinician where you travelled, when you returned, whether you took malaria prevention tablets, whether you had insect bites, and whether anyone else on the trip became unwell.

Fever in hot weather: infection or heat illness?

In hot weather, people sometimes use the word “fever” when they mean they feel overheated. A true fever is usually caused by illness or inflammation, while heat exhaustion and heatstroke happen when the body overheats and cannot cool itself properly.

Heat-related illness may cause:

dizziness
headache
weakness
heavy sweating
muscle cramps
thirst
feeling sick
confusion in more serious cases

Heatstroke is a medical emergency. If someone is very hot, confused, has a seizure, loses consciousness, or does not improve after cooling measures, call 999.

For hot-weather guidance, read Heat Exhaustion and Heatstroke: Symptoms, First Aid and When to Get Help and How to Prevent Heat Exhaustion and Heatstroke in Hot Weather.

How to manage a fever at home

If you have a mild fever and are otherwise well enough to manage at home, the aim is to stay comfortable, hydrated and rested while watching for worsening symptoms.

Helpful steps include:

rest as much as you can
drink fluids regularly
eat light meals if you feel able
wear light clothing
keep the room comfortably cool
avoid alcohol
avoid heavy exercise until you are better
use suitable pain relief or fever medicine if needed

Do not try to cool yourself with ice baths, alcohol rubs or very cold showers. These can make shivering worse and may be unsafe. A comfortable room, light clothing and regular fluids are usually better.

Should you take paracetamol or ibuprofen for a fever?

Paracetamol or ibuprofen may help if fever is making you uncomfortable, causing aches or disturbing sleep. They do not treat the underlying cause, but they can reduce discomfort.

Always follow the dosage instructions on the packet or label. Avoid taking more than the recommended dose, and be careful not to double up with cold and flu products that already contain paracetamol.

Ibuprofen is not suitable for everyone. Ask a pharmacist or clinician first if you have stomach ulcers, kidney disease, some heart conditions, are taking blood thinners, are pregnant, or have been told to avoid anti-inflammatory medicines.

If you are unsure what is safe for you, speak to a pharmacist. You may also find our guide to when to see a pharmacist instead of a GP helpful.

When to see a pharmacist

A pharmacist can help if you have a mild fever with cold symptoms, sore throat, aches, sinus symptoms, hay fever, stomach upset or other common minor illness symptoms. They can suggest suitable medicines and advise whether you should contact a GP or NHS 111.

Speak to a pharmacist before taking over-the-counter medicines if you:

are pregnant
are over 65 and frail
have kidney, liver, heart or stomach problems
take regular prescription medicines
are taking blood thinners
have asthma or have reacted badly to ibuprofen or aspirin before
are unsure which medicine is safe

When to contact a GP or NHS 111

Contact a GP, out-of-hours service or NHS 111 if:

your fever is not improving after a few days
your fever is getting worse despite home care
your temperature is very high, especially around 40°C or above
you feel increasingly weak, lightheaded or unwell
you are peeing much less than usual or your urine is very dark
you have severe thirst or signs of dehydration
you have fever after recent foreign travel
you have a fever with a rash
you have fever with chest pain, shortness of breath or persistent cough
you have fever with severe abdominal pain
you have fever with pain when urinating or back pain
you have a weakened immune system
you are undergoing cancer treatment or take medicines that suppress immunity
you have a long-term condition and are worried
you are not sure what to do

You can use NHS 111 online in England or call 111 if you need urgent advice but it is not a 999 emergency.

If you need help arranging care, read How to Get a GP Appointment Quickly in the UK.

When to call 999

Call 999 or go to A&E if fever comes with symptoms that suggest a serious emergency.

Call 999 if you or someone else has:

difficulty breathing
blue, grey, pale or blotchy skin, lips or tongue
confusion or slurred speech
extreme drowsiness or cannot stay awake
collapse or loss of consciousness
a seizure or fit
severe chest pain
a stiff neck with severe headache or dislike of bright lights
a non-blanching rash that does not fade when pressed
severe, worsening pain
signs of sepsis

Trust your judgement. If someone looks seriously unwell, is deteriorating quickly, or you are frightened by their condition, seek emergency help.

Fever and sepsis: warning signs

Sepsis is a life-threatening reaction to infection. It can happen with many different infections, including chest infections, urinary infections, abdominal infections, skin infections and meningitis.

Fever can be one sign of sepsis, but some people with sepsis have a low temperature instead, especially older adults or people with weakened immune systems.

Possible sepsis warning signs in adults include:

confusion or slurred speech
severe breathlessness
blue, grey, pale or blotchy skin, lips or tongue
extreme shivering or muscle pain
not peeing all day or peeing very little
feeling like you may die
severe drowsiness or being difficult to wake
fast breathing or fast heartbeat

The NHS has detailed information on sepsis symptoms and when to get urgent help. If you suspect sepsis, do not wait.

Fever in older adults

Fever can be harder to recognise in older adults. Some older people do not develop a high temperature even when they have a serious infection. Instead, they may become confused, unusually sleepy, weak, off their food, dizzy, short of breath, more likely to fall, or generally “not themselves”.

Seek medical advice promptly if an older adult has possible infection symptoms, sudden confusion, reduced urination, severe weakness, breathlessness, chest pain, new falls, or is deteriorating quickly.

Fever in people with a weakened immune system

A fever can be more serious if your immune system is weakened. This may include people having chemotherapy, transplant medicines, high-dose steroids, some biologic medicines, advanced HIV, or conditions that affect immune function.

If you have been told you are immunosuppressed, follow the advice from your specialist team. In many cases, a fever should be reported urgently, even if you do not feel severely unwell.

Should you test for COVID or flu?

Testing guidance changes over time and depends on your circumstances, work setting and risk level. In general, if you have fever with cough, sore throat, runny nose, body aches or fatigue, a respiratory virus is possible.

If you are at higher risk of serious illness, work with vulnerable people, or have been advised to test by your workplace or healthcare provider, follow the current guidance relevant to you.

Regardless of testing, it is sensible to avoid close contact with vulnerable people while you have a fever or feel acutely unwell.

What should you eat and drink with a fever?

Fluids are more important than food in the short term. Drink small amounts often if you do not feel like drinking much. Water is fine. Oral rehydration solutions from a pharmacy may help if you have vomiting, diarrhoea or signs of dehydration.

Eat if you feel able. Light foods such as soup, toast, yoghurt, fruit or simple meals may be easier than heavy foods. Do not force large meals if you feel nauseous.

Avoid alcohol while you are unwell, especially if you have fever, dehydration, vomiting, diarrhoea or are taking medicines.

Can fever cause dizziness or fainting?

Yes. Fever can make you sweat, lose fluids and feel weak or lightheaded. If you are not eating or drinking much, dizziness can be worse. Standing up quickly may also make you feel faint.

Get advice if dizziness is severe, you faint, you have chest pain, palpitations, breathlessness, confusion, dehydration, or you are not improving.

How to measure your temperature properly

Use a digital thermometer if possible. Follow the instructions for the thermometer you have, as readings can vary depending on whether it is used in the mouth, ear, armpit or forehead.

Useful tips include:

wait after drinking hot or cold drinks before taking an oral temperature
make sure ear thermometers are positioned correctly
repeat the reading if it seems unusually high or low
write down the temperature, time and any medicine taken
focus on symptoms as well as the number

If the reading does not match how you feel, check again and consider using another thermometer if available.

Frequently asked questions about fever in adults

Is 37.5°C a fever?

Not usually. A temperature of 37.5°C may be normal for some people, especially later in the day, after exercise or in a warm environment. Fever in adults is usually considered around 37.8°C or above, but symptoms and how unwell you feel matter too.

Is 38°C a high temperature in adults?

Yes, 38°C is generally considered a high temperature or fever. If you feel otherwise well, it may be reasonable to rest, drink fluids and monitor symptoms. Seek advice if you feel very unwell, symptoms worsen, or you have red flags.

Is 39°C dangerous in adults?

A temperature of 39°C is a higher fever and should be watched carefully. It is not automatically an emergency if you are otherwise stable, but seek medical advice if it does not improve, keeps rising, or comes with symptoms such as confusion, breathing difficulty, chest pain, severe headache, rash, dehydration or severe pain.

When is a fever too high?

A temperature around 40°C or above is very high and should prompt medical advice, especially if it does not come down or you feel very unwell. Any fever with emergency symptoms such as confusion, breathing difficulty, collapse, blue lips, stiff neck or non-blanching rash needs urgent help regardless of the exact number.

How long should a fever last in adults?

Many fevers caused by viral infections improve within a few days. Contact a GP or NHS 111 if your fever is not improving, is getting worse, or lasts longer than expected, especially if you have other worrying symptoms.

Should I stay in bed with a fever?

You do not have to stay in bed all day, but you should rest and avoid heavy exercise while you are feverish. Try to sleep, drink fluids and return to normal activity gradually as you recover.

Can stress cause a fever?

Stress can make you feel hot, flushed or shaky, but a true fever is more often caused by infection or inflammation. If your temperature is raised and you feel unwell, do not assume it is stress without considering other symptoms.

Can dehydration cause a fever?

Dehydration can make you feel hot, weak, dizzy and unwell, and it can happen alongside fever. In hot weather, dehydration and heat illness may be confused with infection. Seek help if you are very thirsty, peeing little, confused, fainting or getting worse.

Should I take a cold shower for a fever?

A cold shower is usually not necessary and may make you shiver, which can feel worse. Keep the room comfortably cool, wear light clothing, drink fluids and use suitable fever medicine if needed.

Can I go to work with a fever?

It is usually best to stay home while you have a fever and feel acutely unwell, especially if you may have an infectious illness. Avoid close contact with vulnerable people until you are improving and fever-free.

Final thoughts

A fever in adults is usually a sign that the body is fighting an infection. Many fevers improve within a few days with rest, fluids and simple symptom relief. But fever should not be ignored if it is very high, worsening, persistent, linked with recent travel, or comes with symptoms such as confusion, breathing difficulty, chest pain, rash, stiff neck, severe pain or dehydration.

Use a pharmacist for mild symptoms and medicine advice. Contact a GP or NHS 111 if you are worried, symptoms are not improving, or you have risk factors. Call 999 if there are emergency symptoms or possible sepsis.

For official guidance, see the NHS guide to fever in adults, NHS Inform guidance on fever in adults, and NHS information on sepsis.

This article is for general information only and should not replace medical advice. If you are worried about your symptoms or someone else’s, contact NHS 111, your GP, or call 999 in an emergency.