Prolonged fever, or a fever that will not go away
If a fever lasts several days or weeks, or keeps coming back after treatment, the cause needs to be found. This page explains what the doctor asks, which tests may be needed, and why taking antibiotics just to bring the fever down is not the right approach.
What is a prolonged fever?
The body heats up when it fights an infection. A fever is therefore not a disease in itself. It is a sign that something is going on. In the sources, a fever usually means a temperature of 38 degrees or above, although you may feel feverish even at a lower reading.
When a fever lasts for several days, comes back over several weeks, or returns after one or two courses of treatment, the doctor looks for the cause. In medical writing, a fever with a recorded temperature of 38.3 or above on several occasions, whose cause has not been shown by the first tests, is called “fever of unknown origin”. The name sounds scarier than it is. It means we have not yet found the cause. It does not mean the cause is bad.
The causes of a prolonged fever fall into several groups: infections, inflammatory and autoimmune diseases, some tumours, and others such as a drug reaction or a blood clot. Infection is one of the main groups, but not every prolonged fever is an infection.
Who should see a doctor?
It is a good idea to book an appointment for assessment if:
- you have a fever with no obvious cause that lasts several days or is not improving
- the fever goes away and then comes back
- you also have night sweats, weight loss for no reason, joint pain or long-lasting tiredness
- another doctor has referred you to look into a fever
- the fever began after a trip, contact with farm animals, or eating dairy or food that may not have been safe
- the fever started after an operation, or alongside a wound that is not healing
If your body defences are weak (for example because of diabetes, chemotherapy, an organ transplant or medicines that suppress the immune system), take a fever seriously sooner. The sources list people with weak defences as a higher-risk group for serious infections.
How is it investigated?
Your history is the most important step, and sometimes it points the way on its own. The doctor will probably ask:
- when the fever started and what its pattern is. Keeping a temperature record at home helps a great deal
- about travel: where you went, what you ate, where the water came from, and whether you had insect bites
- about animal contact, farms, and unpasteurised milk and cheese
- about contact with anyone who has been ill
- about your job and where you live
- about any new medicines, both prescribed and herbal
- about existing conditions, operations, or any device placed in your body
A careful examination follows: skin, mouth and teeth, heart, abdomen and lymph glands. The doctor chooses the tests and imaging based on your history and examination. This is why two patients with a similar fever may have different tests.
Finding the cause may not happen in one visit and can take several steps. This is normal and does not mean anyone has failed.
What does treatment generally look like?
Treatment depends on the cause. If the cause is a bacterial infection, an antibiotic suited to that bacterium is chosen. If it is a virus, antibiotics do not help and the treatment is different. If the cause is inflammatory or not an infection, it may be necessary to work with another specialist.
An important point is that, in prolonged fever, a “trial” of antibiotics is usually not recommended. The sources say this has no proven benefit and can delay finding the real cause. Only in people who are very immunocompromised or seriously unwell may the doctor start an antibiotic without waiting for results.
What can you do yourself?
- Keep a record of your temperature. Note the time, the reading, where you measured it (under the tongue or under the arm) and any other symptoms. The guide on keeping a fever log at home has an example.
- Rest and drink fluids. The sources recommend plenty of water and rest.
- Stay at home while you have a fever and have less contact with others until it settles.
- Take medicines only on the advice of a doctor or pharmacist. Do not use leftover medicines from before or someone else’s medicines.
- Gather your records. Bring earlier test and culture results and a list of the medicines you have taken so far.
- Mention your travel and any contact with animals. Even if it was some time ago.
When to seek urgent care
Call 115 (Iran’s emergency number) or go to the nearest emergency department if you have any of these along with the fever:
- confusion, or speech that is slurred or makes no sense
- very fast or difficult breathing
- skin that is blue, pale or mottled
- severe shivering you cannot control
- a very high or very low temperature
- a rash or spots that do not fade when pressed
- a stiff neck with a severe headache
- very little urine, or no urine for many hours
For the full list, see the guide on when to seek urgent care. In these situations, do not drive yourself.
Symptoms and treatment plans differ for every patient. Talk to your doctor about your own situation.
Frequently asked questions
How many days of fever should I put up with before seeing a doctor?
There is no magic number. The sources say that if you are looking after the fever at home and it is not getting better, or is getting worse, you should contact a doctor early. The same applies if the fever is very high or will not come down. If you have any warning signs along with the fever, read the guide on when to seek urgent care and do not wait.
Should I take an antibiotic to make the fever go away?
It is better not to. Antibiotics work only on some bacterial infections and do nothing against viruses. The sources say that giving antibiotics without finding the cause has not helped in prolonged fever, and it may even delay finding the real cause. The exceptions are people whose body defences are very weak or who are seriously ill, and the doctor decides that.
Is every prolonged fever an infection?
No. Infections are one of the main groups of causes, but inflammatory diseases, some tumours and other causes can also produce a long fever. Sometimes no cause is found even after a full work-up. That is why a regular, step-by-step investigation is needed, and guessing is not enough.
What kind of thermometer should I use?
The NHS recommends a digital thermometer used under the tongue or under the arm. Forehead strips are not accurate. If you do not have a thermometer and feel feverish, tell the doctor anyway, because you can have a fever even when the reading is below 38.
I developed a fever after a trip. Should I tell the doctor?
Yes, always. An infection picked up while travelling can show up after you come home. Tell the doctor where you went, the dates, what you ate and drank, and any contact with animals or insect bites.