High temperature (fever) in adults

A high temperature, also called a fever, usually means a measured body temperature of 38°C or above. You may still feel feverish when a thermometer reads below 38°C, especially if you feel hot, cold, or shivery.

Fever is a symptom, not a disease. It often happens when the immune system responds to an infection. Many causes are mild and improve within a few days, but the temperature alone cannot show what is causing it or how serious it is.

Body temperature changes during the day and can be affected by exercise, hot surroundings, heavy clothing, and how it is measured. Use a digital thermometer and follow its instructions. Measurements from the mouth, ear, forehead, or armpit may differ slightly.

Along with a raised thermometer reading, you may have:

• Chills or shivering
• Sweating or hot, flushed skin
• Headache or body aches
• Tiredness and weakness
• Reduced appetite
• Thirst or a dry mouth
• A faster heartbeat

Other symptoms may point towards the cause. A cough, blocked nose, or sore throat may occur with a respiratory infection. Vomiting or diarrhoea may suggest a stomach infection. Pain when passing urine can occur with a urinary infection.

How you feel matters as much as the number. A person with a modest fever who is confused or struggling to breathe needs more urgent help than someone with a higher reading who is alert, drinking, and improving.

The most common causes are short-term infections. These include colds, influenza, COVID-19, throat infections, and stomach infections. Urinary infections and some skin or dental infections can also cause fever.

Less commonly, fever may be related to an inflammatory condition, a reaction to a medicine, or an infection lasting longer than expected. Recent travel, an insect or tick bite, surgery, dental treatment, or contact with an infectious illness may be relevant.

A high body temperature caused by overheating is different from an infection-related fever. Heat illness can follow strenuous activity or time in a very hot environment. Confusion, collapse, or hot skin after heat exposure can indicate heatstroke, which is an emergency.

Serious infections can also cause fever. These include pneumonia, meningitis, and infections that trigger sepsis, the body’s extreme response to an infection. These are less common, and they usually cause other concerning symptoms.

Call emergency services now if a fever occurs with:

• Severe difficulty breathing, blue or grey lips, or severe chest pain
• New confusion, collapse, extreme drowsiness, or difficulty waking
• A seizure
• A severe headache with a stiff neck or strong light sensitivity
• A rash that does not fade when pressed with a clear glass
• Confusion, fainting, or loss of coordination after heat exposure

Seek same-day medical advice if you have a temperature of 38°C or above while receiving chemotherapy, after a transplant, or when a condition or medicine severely weakens your immune system. Follow any temperature plan given by your specialist team, as it may use a different threshold.

Same-day advice is also appropriate if you are rapidly worsening, cannot keep fluids down, are passing very little urine, or have severe pain. Contact a clinician promptly after recent surgery if fever develops with increasing wound pain, redness, swelling, or discharge.

Book a routine appointment if the fever lasts several days without improving, keeps returning, or has no clear explanation. Arrange follow-up if it comes with ongoing night sweats, unexplained weight loss, or other persistent symptoms.

A clinician may ask when the fever began, how it was measured, and whether it comes and goes. They may ask about cough, pain, rashes, stomach symptoms, urinary symptoms, recent travel, animal or insect exposure, new medicines, and contact with illness.

An examination may include checking your breathing, heart rate, hydration, throat, ears, skin, chest, and abdomen.

Tests are not always needed. Depending on your symptoms and risk factors, testing may include a urine test, a respiratory swab, blood tests, or a chest X-ray. The aim is to find the cause rather than simply confirm that the temperature is raised.

Treatment depends on the cause. Antibiotics like amoxicillin or doxycycline only treat certain bacterial infections; they do not treat viral infections such as colds or influenza.

While recovering:

• Rest and drink enough fluid to keep your urine pale
• Wear light, comfortable clothing and keep the room comfortably cool
• Avoid strenuous exercise until the fever and weakness have settled
• Eat if you feel able, without forcing food
• Stay home while you have a high temperature and feel unwell

Paracetamol or ibuprofen may help if fever, headache, or body aches are making you uncomfortable. Follow the package directions. Check cold and flu products so you do not accidentally take more than one medicine containing paracetamol. Ask a pharmacist or clinician before using these medicines if you are pregnant, take other medicines, or have a relevant long-term condition.

Do not try to cool yourself with ice baths or very cold showers. These can trigger shivering and make you feel worse. If overheating rather than infection may be responsible, move to a cool place and seek urgent help for confusion, collapse, or other signs of heatstroke.

Many fevers caused by short viral infections improve within a few days. The temperature may rise and fall during that time, and sweating can occur as it comes down.

Recovery depends on the cause. Symptoms such as tiredness or cough can remain after the fever settles. Keep drinking, return to normal activity gradually, and seek medical advice if you worsen after initially improving.

If you need an appointment, a record of temperature readings, symptoms, and medicines taken can be useful. Do not rely only on the thermometer reading; changes in breathing, alertness, hydration, and pain are also important.

Is 37.5°C a fever?

A temperature of 38°C or above is commonly used as the fever threshold in adults. However, your usual temperature, the time of day, and the measurement method can affect the reading. You can still feel feverish below 38°C.

Should I try to “sweat out” a fever?

No. Heavy clothing and blankets can make you overheated and dehydrated. Wear comfortable layers and adjust them if you become hot or shivery.

Is a higher number always more serious?

No. The temperature is only one part of the picture. Your symptoms, medical history, immune system, and likely cause are also important. Confusion or breathing difficulty is concerning at any temperature.

Can fever return after medicine wears off?

Yes. Fever-reducing medicine may lower the temperature temporarily without treating the cause. Seek advice if the fever persists, repeatedly returns, or is accompanied by worsening symptoms.