Fever in adults

Normally, your body temperature is about 37°C but this can vary from person to person and at different times of the day. When your core temperature rises above 38°C it’s described as a fever or high temperature. If your temperature is 39°C or over it’s considered to be a high fever. You can use a thermometer to see how high you temperature is. Read more about thermometers and how to use them.

You can often get a fever when you have a cold or flu virus, but there are other reasons why your temperature might be high. If you have a fever you usually feel unwell and can feel hot and sweaty or cold and clammy.

This page is about fever in adults. Read more about fever in children.

A fever usually lasts 3 to 5 days, and most people recover by managing their symptoms at home. However, there are times when you should see a healthcare provider urgently for fever (see when to seek help below). 

• Drink plenty of fluids, especially water or rehydration fluid (little and often is best). You sweat more when you have a fever. Drink enough so your urine (pee) is light yellow and clear. Read more about dehydration.
• Avoid alcohol as it can make yo more dehydrated.
• Rest while you recover.
• Open a window for ventilation but avoid draughts.
• Wear lightweight comfortable clothing and use lighter bedding. Don't use hot water bottles or electric blankets.
• Use a cool cloth to wash your face, hands and neck.
• Change bed linen and clothing regularly, especially if they are wet from sweat.

Although most fevers will settle down in a few days and are not worrying, you sometimes need to seek medical advice.

Note: Older adults may not get a fever even if they have a serious infection. If an older adult is not eating or drinking or is confused, seek medical advice even if they don't have a fever.

When to contact a healthcare provider or call Healthline 0800 611 116 for advice  

If you have a fever and: 

• your urine is darker than usual 
• it lasts for more than 3 days or you seem to be getting sicker 
• you’re shivering or shaking uncontrollably, or have chattering teeth, and it doesn’t stop within an hour or so 
• you have a bad headache that doesn’t get better after taking pain medicines 
• you’re having trouble breathing 
• you’re getting confused or you’re unusually drowsy 
• you have recently travelled overseas. 

If you have a fever and you are: 

• being treated for immune deficiency 
• on immune-suppressant drugs, such as regular steroids, methotrexate, azathioprine or cyclophosphamide 
• taking medication that can affect your immune system 
• having, or have recently completed, treatment for cancer, leukaemia or lymphoma 
• a transplant recipient 
• HIV positive. 

If you’re pregnant and have a fever, check with your midwife, doctor or nurse before you take any medicines. If your fever lasts for longer than a day, talk to your lead maternity carer (LMC). 

A viral infection such as a cold, flu, COVID-19 or another respiratory tract infection is the most common cause of a fever. A bacterial infection, such as tonsillitis or pneumonia, is a less common cause but can be more serious.  

When you have an infection your body's natural reaction is to raise your body temperature to help kill off the virus or bacteria causing the infection.  

Other causes of fever include:

• vaccination
• allergy
• drug reaction
• hyperthyroidism
• heat stroke
• some cancers
• inflammatory diseases, such as connective tissue diseases, inflammatory arthritis, inflammatory bowel disease
• tissue damage, eg, from a crush injury or surgery

Medicine 

As a fever is your body’s way of fighting infection, medicine isn’t needed for mild fever. However, you can take paracetamol if you also have a headache or muscle aches or pains. If your fever is due to a bacterial infection, you may need antibiotics to kill the bacteria, but antibiotics won’t help with a fever caused by a viral infection.  

Apps reviewed by Healthify

You may find it useful to look at some symptom checkers.