Fever is a natural defence mechanism of the body. It’s often benign, but sometimes reveals an underlying problem. Find out how to recognize a fever, how to treat it effectively at home, and when to seek advice from a health care professional, such as your pharmacist.

Understanding fever

In general, our body temperature fluctuates between 36°C and 37.5°C. But you should know that this value changes throughout the day, often rising by approximately 0.5°C in the afternoon. Fever in adults occurs when body temperature is unusually high; it is often accompanied by common symptoms such as chills or fatigue. It’s as if the body starts playing “defence” to fight infection or inflammation.

What temperature is considered a fever in adults?

According to the Institut universitaire de cardiologie et de pneumologie de Québec, although there is no definitive threshold, most health care professionals consider any value higher than 38°C to indicate the onset of a fever. But there’s no need to panic—a fever doesn’t necessarily mean the situation is serious.

What causes fever?

Fever is not a disease in and of itself, but rather a weapon used by your immune system.

The mechanism of fever

When a virus or bacteria enters your body, the hypothalamus, which is the temperature control centre in your brain, orders the body’s temperature to rise. In essence, the body is turning up the heat in order to slow down the multiplication of microbes and give your natural defences a boost. This is what causes shivering, an accelerated heart rate, and of course, the sensation of heat.

Recognizing the symptoms

A thermometer is the best way to confirm a fever, but there are other telltale signs you can keep an eye out for, too:

• Shivering or chattering of the teeth;
• Excessive sweating;
• Feeling very warm;
• Sore muscles or a headache;
• Severe fatigue;
• Loss of appetite.

Influenza is one of the most common causes of fever in adults. Its symptoms come on rapidly.

How to take your temperature

Here’s how to take your temperature correctly using different methods, plus a few tips on getting a reliable reading.

Oral temperature: How to take it correctly

• Wait at least 15 minutes after drinking or eating.
• Place the tip of the thermometer under your tongue, facing backwards, and close your mouth.
• Wait for the beep (usually one minute) to get an accurate temperature reading.

Axillary temperature: When to use this alternative

• Use this method when the other options would cause too much discomfort.
• Wipe the area under the arm well before placing the thermometer in the centre of the armpit.
• Keep your arm close to your body throughout the process.

Rectal temperature: The most accurate reading

• Apply a lubricant to the tip of the thermometer.
• Gently insert the tip to a depth of around 2 cm.
• This method offers the most reliable measurement.

Body temperature reference chart

What are the different types of fever in adults?

Infection: The most common culprit

Most of the time, fever in adults is caused by a viral or bacterial infection.

• Common viral infections: cold, flu, gastroenteritis;
• Common bacterial infections: urinary tract infection, pneumonia, throat infection.

Influenza: Specific symptoms

The flu is characterized by a sudden high fever, intense aches and pains, and a dry cough, often accompanied by chills and fatigue.

Other possible causes

• Reaction to a drug or vaccine;
• Inflammatory disease such as rheumatoid arthritis;
• Hyperthermia: increased body temperature caused by heatstroke or intense physical effort (the elderly are particularly at risk for this).

How to reduce fever in adults

At home:

• Get plenty of rest. Your body needs its strength to fight the infection.
• Stay hydrated. Drink plenty of water, broth, or juice to compensate for fluid loss due to perspiration.
• Dress lightly. Avoid thick, heat-trapping blankets.

Acetaminophen vs. ibuprofen: Which is better?

• Acetaminophen and ibuprofen are both excellent choices for fever reduction and pain relief.
• Ibuprofen has the advantage of also being an anti-inflammatory, and it remains effective for slightly longer than acetaminophen (6 to 8 hours vs. 4 to 6 hours).
• Before taking any medication, ask your pharmacist for advice, especially if you’re taking other medications or have health problems.

Rubbing alcohol: A common misconception

Even if might feel good in the moment, rubbing alcohol on the skin is not recommended. Alcohol can be absorbed through the skin, causing irritation and even intoxication.

How long does a fever last?

It depends on what’s causing it. In the case of influenza, fever lasts for an average of 3 to 5 days. With other viruses, it generally resolves within a week. If your fever persists for more than 72 hours, recurs frequently, or worsens over time, consult a doctor or other health care professional.

Symptoms to watch out for

Call Info-Santé 811 or make an appointment with your doctor if your fever is accompanied by any of the following symptoms:

• Difficulty breathing;
• Confusion, abnormal drowsiness, convulsions;
• Stiff neck or intense headaches;
• Severe vomiting or signs of dehydration;
• Very high fever (more than 39.5°C);
• Chest pain or rash.

When should I go to the emergency room for a fever?

We recommend going to the emergency room if you have any of the following symptoms:

• Severe respiratory distress;
• Intense chest pain;
• Loss of consciousness;
• Convulsions;
• Stiff neck with a high fever;
• Severe dehydration.

Get vaccinated early

Your Brunet-affiliated pharmacist-owner is one of the many health specialists who can advise you. Fever is best managed at home with rest and proper care, but when in doubt, don’t hesitate to come and see us. We’re here to help.

Other articles that might interest you

Fever and pain in children

Cold or flu? 9 ways to tell them apart

The basics on COVID-19

Tylenol (acetaminophen), pregnancy, and autism: Everything you need to know

Last updated on December 4, 2025