Have a Fever Over 100? Here’s What to Do About Fever in Adults

A fever is defined as a body temperature above 100.4°F (38.3°C).  The normal oral temperature for a resting, healthy adult is about 98.6°F (37°C). However, what is considered a “normal” body temperature can vary by one degree based on age, race, and other factors.  For example,  an individual who is over 70 years old may have a lower normal temp at 96.8°F (36°C). Furthermore, a person’s temperature may increase or decrease by 1 to 2 degrees throughout the day depending on time of day, activity level, the environment (hot or cold), hydration status, or even medications that he/she may be taking.

In general, a fever indicates the presence of inflammation.  It may occur in the presence of an underlying illness such as an infection, malignancy, or even bodily injury.  It may also be associated with certain autoimmune diseases.  Less commonly, certain medications are associated with fevers.  A fever itself, however, is merely a symptom, and not a disease.

A brain structure called the hypothalamus sets body temperature, and produces a fever response.  It can raise the body’s internal thermostat to combat illnesses.  In this way, the hypothalamus responds to an infection or inflammation by helping the immune system to defeat the offending agent.  Although it may not be comfortable, a temperature of up to 102°F is generally safe in adults. In fact, most healthy adults can even tolerate a fever as high as 103°F to 104°F for short periods of time without having any significant problems.  The body temperature usually returns to normal once the illness resolves.  Click here for a list of fever symptoms in adults.

What is the difference Between Fever and Hyperthermia?

The terms fever and hyperthermia are commonly confused, and are completely different entities. Hyperthermia is defined as a sustained body temperature above 104°F (40°C), and is just another word for overheating.   The hypothalamus functions normally, but the body’s ability to get rid of heat is impaired.  This most commonly occurs during exposure to extreme heat such as when exercising in hot outdoor temperatures.   Insufficient hydration can also increase the risk of hyperthermia.  

Ranges in Body Temperatures

While there are numerous definitions of fever or pyrexia, the CDC defines a fever as a temperature above 100.4°F (38°C). However, it is important to keep in mind that, in the elderly, temperatures below 100.4 may also be indicative of a fever.  The hypothalamus of these individuals is less able to elevate their body temperatures.  It is, therefore, important to take note of any additional concerning symptoms.

Here are some other definitions related to body temperature: 

  • Normal: temperatures between 97.7°F (36.5°C) and 99°F (37.2°C)
  • Low-grade fever: temperatures between 99°F (37.2°C) and 100.4°F (38°C)
  • Fever (pyrexia): temperatures between 100.4°F (38°C) and 105.8°F (41°C)
  • Hyperpyrexia: temperatures between 105.8°F (41°C) and 109.4°F (43°C)
  • Temperatures above 109.4°F (43°C) are usually fatal

Symptoms of Fever

Fevers can cause a wide variety symptoms. The most common ones are listed below:

    1. Muscle aches
    2. Headaches
    3. Irritability
    4. Chills
    5. Loss of appetite
    6. Sweating
    7. Generalized weakness

Click here for a list of fever symptoms in adults.

How to Take Your Temperature – Fever in Adults

Adults should use a digital thermometer, placing the tip under the tongue.  Forehead (temporal artery) measurements are also fairly accurate.  Readings taken with an ear (tympanic membrane) thermometer can vary, and may be inaccurate if there is an ear infection.  When readings from both ears are compared, the numbers may differ.

Taking a temperature under the armpit is not very accurate, but can be a quick way to take one’s temperature. When doing so, add one degree to know the true core body temperature.

How to Take Your Temperature – Fever in Children

When taking a child’s temperature, the digital thermometer tip should be placed under the tongue if age four or older. Tympanic membrane and forehead models may be used over the age of six months, but expect some variation in readings when using ear thermometers. 

In infants, the most accurate way to take a temperature is rectally.  A fever in infants under age three months can be a sign of a life threatening infection, so taking the temperature correctly is crucial.   

How to Take Your Temperature – Thermometers

Wash your hands with soap and warm water prior to using any thermometer. 

Mercury Thermometer

Because digital thermometers have become very affordable, the use of old glass mercury thermometers should be avoided.

Mercury is poisonous and is released when a glass thermometer breaks.  Furthermore, because it takes some skill to get a proper temperature reading, so they are not as accurate as the latest digital thermometers.

What to Do if a Mercury Thermometer Breaks.

Digital Thermometers

Digital thermometers are affordable, and readily available nowadays.  Because of the digital number displays, they are easy to read.  Click here for thermometer recommendations.

Before each use, make sure that the thermometer is clean and turned on. Again, wash your hands with soap and warm water to prevent the spread of infection. Follow the manufacturer’s instructions regarding appropriate usage for an accurate temperature reading, and always clean the thermometer before putting it away.

Fever in adults – How to Treat a Fever

  • Stay hydrated.  In response to a fever, the body sweats in an effort to cool off.  Since sweat contains water, it must be replaced.  Water is the best option for rehydration in most cases.  If there are fluid losses due to vomiting or diarrhea, electrolyte replacement beverages may be ideal.
  • Regularly take and record your temperatures.  If taking medication to lower the fever, it should drop within an hour.
  • Monitor associated symptoms.  Take your temperature more frequently if your symptoms change (e.g. if you start vomiting)
  • Antipyretics.  For fevers that are uncomfortable (e.g. associated muscle aches, headaches) you can take acetaminophen or ibuprofen. These medications help to reset your body’s thermostat, and lower the temperature.
  • Do not give aspirin or products that contain it to children or teens under the age of 20. This can potentially cause Reye syndrome, a potentially dangerous condition
  • Watch for signs of dehydration.  This can occur if the fever causes you to sweat excessively, or is associated with vomiting or diarrhea.  Signs of dehydration include thirst, dry skin, dry mouth, chills, feeling tired or weak, and dark-colored urine.
  • Other fever remedies for adults.

When to Call Your Doctor

  • Fevers above 103°F 
  • Persistent fever.  Many viral illnesses, especially the flu, cause fevers of 102°F or higher for three to four days.  If associated with such illnesses, it is worth seeing a doctor for any fever that lasts longer.  For fevers that develop with no other symptoms, one should see a doctor if it lasts more than 48 hours.
  • If your fever is associated with: 
    • Shortness of breath, chest pain 
    • Severe cough 
    • Seizures 
    • Confusion 
    • Persistent vomiting or diarrhea 
    • Severe headache 
    • Skin rash
    • Sensitivity to bright light and/or neck stiffness (could indicate meningitis – an infection of the lining of the brain, spinal cord)
    • Severe abdominal pain (could indicate diverticulitis, appendicitis, or other abdominal disorder)
    • Pain with urination (could indicate a urinary tract infection)

Treatment of Hyperthermia 

Hyperthermia is different from just a fever.   It is more dangerous.  The body becomes overheated and loses its ability to thermoregulate.  If measures are not taken to cool down the body, it can lead to organ damage and death.  When the body’s temperature rises above 104°F and is associated with other symptoms, it is called heatstroke.  This is a medical emergency and should be treated as such. Call 911. Symptoms may include:

  • Confusion 
  • Vomiting 
  • Slurred speech
  • Racing heart
  • Rapid breathing 
  • Loss of consciousness

Before these symptoms occur, moving indoors to an air-conditioned room can gradually lower the core body temperature to a safe range.  Drinking plenty of cool fluids is also helpful.

Fever in Newborns and Children

Fevers in newborns and children are often treated differently than those seen in adults. Please refer to the following links:

Fever in newborns can be serious, and should not be ignored.

Fever in Young Children (Toddlers) 

Remedies for fever in children


Note that the information in this article is purely informative and should never be used in place of the advice of your treating physicians.

Leah Alexander, MD, FAAP
Leah Alexander, MD, FAAP

Dr. Alexander began her pediatric career at Elizabeth Pediatric Group of New Jersey in 2000, and has practiced at Pediatricare Associates of New Jersey since 2005. After graduating from Kalamazoo College and Michigan State University College of Human Medicine, she completed her pediatric training at Overlook and Morristown Memorial Hospitals. She is board certified in General Pediatrics. In addition to pediatrics, Dr. Alexander pursued her interest the culinary arts with study at the French Culinary
Institute. In 2007, she opened Global Palate, LLC, catering small group events for six years. Dr. Alexander has also been a professional writer and editor since 2018, engaging in a variety of medical editing and writing projects.