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°C). A normal oral temperature for a resting, healthy adult is about 98.6°F (37°C). However, what is considered “normal” can vary based on age, race, and other factors e.g. in an individual > 70 years old, a normal temp could be lower at 96.8°F (36°C). Furthermore, a person’s temperature can go up or down 1 to 2 degrees throughout the day and vary depending not only on time of day but also based on activity level, one’s environment (hot, cold, etc.), hydration status, or even medications that he/she may be taking.

Fever is a sign of inflammation, meaning that it most commonly indicates that there is an underlying illness (such as an infection: cold, etc.) or even a bodily injury. It may also be associated with certain autoimmune diseases. Less commonly, it may be caused by certain medications that one is taking. A fever, in and of itself, however, is not a disease.

A fever appears to be one way your body fights illness i.e. your body raises its temperature via its internal thermostat (located within a portion of the brain called the hypothalamus). It does this in response to infection or inflammation to help your 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. Body temperatures usually return to normal once the illness goes away. Click here for a list of fever symptoms in adults.

What is the difference Between Fever and Hyperthermia?

Fever and hyperthermia are commonly confused for one another. Hyperthermia is just another word for overheating. Basically, the body is working fine (i.e. without infection) and the thermostat has not been raised (as with fever), but for some reason, the body’s ability to get rid of heat is impaired. This most commonly occurs when one is vigorous exercising in hot temperatures. You are also more at risk if you are not well hydrated. 

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 certain populations, like the elderly, lower temperatures than 100.4 may also be indicative of a fever, as the elderly are not able to raise their body temperatures as easily as the young.

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 of different symptoms. The most common symptoms 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 to take their temperature under the tongue. Tympanic membrane (ear) and forehead (temporal artery) measurements are also fairly accurate. 

Taking a temperature under the armpit is not very accurate but can be a quick way to take one’s temperature. 

How to Take Your Temperature – Fever in Children

When taking a child’s temperature, a digital thermometer should be placed under the tongue. Tympanic membrane (ear) and forehead measurements are also fairly accurate. 

In infants, identifying the most accurate temperature can be very important. As such, rectal temperatures should be taken. This is actually the most accurate way to take a temperature. 

How to Take Your Temperature – Thermometers

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

Mercury Thermometer

With how affordable digital thermometers are nowadays, use of your old mercury thermometers should be avoided.

Mercury is poisonous and is risk at being released when a glass thermometer breaks. Furthermore, 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. They are accurate and easy to read as they have a digital display for numbers.

Click here for a list of good thermometers. Before using, make sure the thermometer is clean and turned on. Again, wash your hands with soap and warm water. Follow the manufacturers instructions regarding appropriate and accurate usage. Clean the thermometer before you put it away.

Fever in adults – How to Treat a Fever

  • Stay hydrated. Water is best. Your body may try to cool itself by sweating and you should replace this loss of fluid.
  • Take your temperature regularly. Keep track of what it is. 
  • Take your temperature more frequently if your symptoms change e.g. if you start vomiting
  • 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 drive temperatures down.
  • Do not give aspirin or products that contain aspirin to children or teens under the age of 20. This can potentially cause Reye syndrome, a potentially dangerous condition
  • Watch for signs of dehydration. Dehydration can happen if the fever causes you to sweat excessively, have 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

  • If your fever is over 103°F 
  • Persistent fever. Many viral illnesses, especially the flu, cause fevers of 102°F or higher for short periods of time (up to 12 to 24 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 gastroenteritis, appendicitis, or other abdominal infection)
    • Pain with urination (could indicate a urinary tract infection)

Treatment for Hyperthermia 

Again it is important to remember that hyperthermia is not the same thing as a fever. Again, as it is the result of overheating, in most cases it will return to normal after you have rested for an hour or so in a cool room. Make sure to drink plenty of fluids and to avoid exercising too vigorously outside on very hot days. 

Hyperthermia can be more dangerous than a fever. In particular, if your temperature rises above 104°F and is associated with other symptoms, this is called a 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 

Fever in Newborns and Children

Fevers in newborn and children are often treated differently than fevers 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.