Antibiotics are medicines that fight infections caused by bacteria. They are one of the most important tools in modern medicine — but they only work against bacteria, not viruses. Understanding how antibiotics work, when to use them, and what side effects to watch for can help you stay safe and get the most from your treatment. This guide covers everything everyday Canadians need to know about antibiotics, including how your family doctor or walk-in clinic can help you use them wisely.
What Are Antibiotics?
Antibiotics are substances — either natural or synthetic — that target and destroy harmful bacteria in the body. They have been used in medicine for decades and have saved countless lives around the world. However, they are not a cure for everything. For example, they will not help with the common cold or the flu, both of which are caused by viruses.
There are dozens of different antibiotics available today. They are grouped into more than ten families, each with its own way of attacking bacteria. Your doctor will choose the right one based on the type of infection you have and your personal health history.
In Canada, antibiotics are prescription medications. You cannot buy them over the counter at a pharmacy. This rule exists to protect your health and to help slow the growing problem of antibiotic resistance. Health Canada provides guidance on the responsible use of antibiotics for both patients and healthcare providers.
How Antibiotics Work Against Bacteria
Not all antibiotics work the same way. Doctors look at several key features when choosing the right antibiotic for your infection.
Spectrum of Action
Every antibiotic has what is called a “spectrum of action.” This is simply the list of bacteria that the antibiotic can fight. Some antibiotics have a broad spectrum — meaning they work against many different types of bacteria. Others have a narrow spectrum and target only specific bacteria.
When bacteria are not affected by a particular antibiotic, they are called “resistant.” This is known as natural resistance. Therefore, it is important that your doctor identifies the right antibiotic for your specific infection.
In some cases, your doctor may take a sample — for example, from a wound or your throat — and send it to a laboratory. The lab tests the bacteria against several antibiotics to find which one works best. This test is called a culture and sensitivity test. As a result, your treatment can be tailored exactly to the bacteria making you sick.
Bacteriostatic vs. Bactericidal Antibiotics
Antibiotics fight bacteria in two main ways. Some antibiotics are bacteriostatic — they stop bacteria from multiplying, giving your immune system time to finish the job. Others are bactericidal — they directly kill the bacteria.
Your doctor will choose between these types based on the severity of your infection and your overall health. In serious infections, bactericidal antibiotics are often preferred.
How Antibiotics Travel Through the Body
Once you take an antibiotic, your body absorbs and distributes it through your tissues and organs. Different antibiotics travel differently through the body. Some can enter cells easily, while others work best in the bloodstream.
This affects how antibiotics are given. Some antibiotics work well as a pill or liquid taken by mouth. Others need to be given by injection, especially in hospital settings. Some are applied directly to the skin as a cream or ointment. In addition, some antibiotics are filtered out through the kidneys, while others are processed by the liver. This matters if you have kidney or liver disease.
Choosing the Right Antibiotic
Your doctor considers many factors before prescribing antibiotics. The choice is never random — it is based on careful medical judgement.
Key factors include:
The location of the infection (throat, lungs, urinary tract, skin, etc.)
The severity of the infection
Your allergy history, especially any past reactions to antibiotics
Whether you have kidney or liver problems
The results of any laboratory tests on the bacteria causing your infection
Sometimes, doctors need to combine two or more antibiotics. This is more common in serious or complex infections. However, combining antibiotics is always a medical decision — never something to do on your own.
The Mayo Clinic offers a helpful overview of how antibiotics are prescribed and used in everyday medical practice.
Side Effects of Antibiotics
Like all medicines, antibiotics can cause side effects. Most are mild, but some can be more serious. It is important to know what to watch for.
Common Side Effects
The most common side effects of antibiotics include nausea, stomach upset, and diarrhoea. Diarrhoea often happens because antibiotics do not only kill harmful bacteria — they also disturb the healthy bacteria living in your gut. This imbalance in your gut flora can cause digestive problems that last for a few days.
Furthermore, antibiotics can sometimes trigger the growth of fungi, because the healthy bacteria that normally keep fungi in check are reduced. The most common result is a yeast infection (also called candidiasis), which can affect the mouth or genital area. Your doctor can treat this easily if it happens.
Allergic Reactions
Some people are allergic to certain antibiotics. Signs of an allergic reaction include skin rashes, hives, or itching. In rare cases, a severe reaction called anaphylaxis can occur. This is a medical emergency.
If you have had an allergic reaction to one antibiotic in a family — for example, penicillin — you may also react to other antibiotics in the same family. Always tell your doctor or pharmacist about any past antibiotic allergies before starting a new prescription.
Tolerance and Toxicity
How well your body tolerates an antibiotic depends on the medicine’s toxicity level and your individual sensitivity to it. For example, some antibiotics can affect the kidneys or liver if taken in high doses or for a long time. Your doctor will monitor this if needed, especially in older adults or people with existing health conditions.
Antibiotic Resistance: A Growing Concern for Canadians
One of the biggest challenges in modern healthcare is antibiotic resistance. This happens when bacteria change and develop the ability to survive despite antibiotic treatment. As a result, infections that were once easy to treat can become very difficult — or even impossible — to cure.
Resistance develops when bacteria produce enzymes that block the antibiotic’s effect. For example, some bacteria produce an enzyme called beta-lactamase. This enzyme breaks down certain antibiotics — known as beta-lactams — before they can do their job.
How Resistance Spreads
Resistant bacteria do not stay in one place. They can spread from person to person, or even through communities, in a way that resembles an outbreak. This is especially concerning in hospitals and long-term care facilities.
Antibiotic resistance is made worse when antibiotics are used too often or incorrectly. For example, taking antibiotics for a viral infection — like a cold — does nothing to treat the virus. However, it does expose your body’s bacteria to the antibiotic, giving them a chance to develop resistance.
What You Can Do
The most important things you can do to fight antibiotic resistance are simple:
Only take antibiotics when prescribed by a doctor
Always finish the full course of antibiotics, even if you feel better
Never share antibiotics with others
Never save leftover antibiotics for later use
Ask your doctor if an antibiotic is truly necessary for your illness
The World Health Organization explains the global impact of antimicrobial resistance and why protecting the effectiveness of antibiotics matters to everyone.
When to See a Doctor About Antibiotics
If you think you have a bacterial infection, the right first step is to contact your family doctor. Your family doctor knows your health history and can quickly decide whether antibiotics are the right treatment for you.
If your regular doctor is not available, a walk-in clinic is an excellent option. Walk-in clinics across Canada can assess your symptoms, run tests if needed, and prescribe antibiotics when appropriate. Most provincial health plans cover these visits, so there is no reason to delay getting care.
Seek care right away — or go to an emergency room — if you notice any of the following:
A high fever that does not come down
Signs of a severe allergic reaction (swelling of the face or throat, difficulty breathing)
Symptoms that are getting rapidly worse
Severe stomach pain or bloody diarrhoea while taking antibiotics
Confusion or extreme weakness alongside an infection
Remember, never take antibiotics that were prescribed for someone else. And always let your doctor or pharmacist know about every medicine — including vitamins and supplements — that you are already taking, as some can interact with antibiotics.
As always, this article is for general information only. Please speak with your doctor or a qualified healthcare provider before starting, stopping, or changing any medication, including antibiotics.
Frequently Asked Questions About Antibiotics
Can antibiotics treat a cold or the flu?
No — antibiotics only work against bacteria, not viruses. Colds and the flu are caused by viruses, so taking antibiotics for these illnesses will not help. In fact, doing so can contribute to antibiotic resistance over time.
What happens if I stop taking antibiotics early?
Stopping antibiotics before finishing the full course can allow some bacteria to survive and multiply. These surviving bacteria may become resistant to that antibiotic, making your infection harder to treat. Always complete the full course your doctor prescribed, even if you feel better.
Can antibiotics cause a yeast infection?
Yes, antibiotics can cause yeast infections by disrupting the balance of healthy bacteria in your body. Without these beneficial bacteria, fungi like Candida can grow more freely. Talk to your doctor or pharmacist if you develop symptoms of a yeast infection while taking antibiotics.
Are antibiotics covered by provincial health plans in Canada?
The doctor’s visit to get a prescription for antibiotics is generally covered by provincial health plans across Canada. However, the cost of the antibiotic itself depends on your province and whether you have private or public drug coverage. Many provinces have pharmacare programmes that cover the cost of common antibiotics for eligible residents.
What should I do if I have an allergic reaction to antibiotics?
If you experience mild symptoms like a rash or itching after taking antibiotics, contact your doctor or visit a walk-in clinic right away. If you have trouble breathing, swelling of the face or throat, or feel faint, call 911 immediately — this could be a severe allergic reaction called anaphylaxis. Always inform future healthcare providers about your antibiotic allergy.
How do I know if my infection needs antibiotics?
Only a doctor can determine whether your infection requires antibiotics. Your family doctor or a walk-in clinic can assess your symptoms, and may take a swab or urine sample to identify the bacteria causing your infection. Antibiotics are only effective against bacterial infections, so your doctor will make sure the treatment matches your diagnosis.
Key Takeaways
Antibiotics fight bacterial infections — they do not work against viruses like colds or the flu.
There are many families of antibiotics, each suited to different types of bacteria and infections.
Common side effects include nausea, diarrhoea, and yeast infections. Allergic reactions are possible and should be reported to your doctor.
Antibiotic resistance is a serious and growing problem. Using antibiotics correctly helps protect everyone.
Always finish your full course of antibiotics, even if you start feeling better.
In Canada, antibiotics require a prescription. Your family doctor or local walk-in clinic can assess whether you need them.
If you are unsure, always ask your doctor or pharmacist — they are your best resource for safe antibiotic use.
