Why Antibiotics Don't Work on Colds

Photo: fizkes (Shutterstock)

If you’ve been sick with a cough and a sniffly nose for a few days, you might show up at the clinic desperate for something that will make you feel better. But if all signs point to your cold being an ordinary virus (or even a nasty virus like COVID or flu), antibiotics won’t help. Here’s why.

Viruses aren’t bacteria

We need a teensy biology lesson here, but I’ll keep it brief. You know how your body is made of cells? Microscopic bags of fluid that each contain DNA, a mitochondrion (“the powerhouse of the cell”) and so on? Well, bacteria are free-living creatures that each have one cell, and that cell is smaller than most of ours. (Fun fact: They have DNA but no mitochondria.) Some illnesses are caused by bacteria. Salmonella, gonorrhea, Lyme disease, and strep throat are all bacterial.

But then there’s another, smaller type of germ. These are the viruses. They’re far, far smaller than bacteria, and technically they’re so minimalist in the way they’re built that they aren’t even considered to be cells. Viruses are small enough that some of them can make bacteria sick.

We also have human viral illnesses: COVID, flu, RSV, measles, HIV, and rabies are all caused by viruses. By the way: There’s no simple way to remember which illnesses are bacterial and which are viral. You just kind of have to remember (or look up) each one. But most illnesses we call “colds” are viral.

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Antibiotics only work on bacteria

What we call “antibiotics” are usually actually antibacterial drugs. Penicillin, amoxicillin, Z-Pak, Doxycyline, and Ciprofloxacin are all examples of common antibacterial medications.

Each antibacterial drug works because it targets something specific in the bacterium, such as interfering with a process that bacteria require to live and reproduce but we do not. (Obviously, if a drug interfered with our own bodily processes, it would harm us, too. This is why chemotherapy for cancer is so brutal: We’re poisoning our own human cells.)

So, for example, penicillin breaks down bacterial cell walls. Tetracycline binds to ribosomes—protein-making enzymes—but only the form that certain bacteria use. If you have a viral illness, these antibacterial medications won’t do anything to the virus.

Why doctors don’t want to give you antibiotics “just in case”

When you go to the urgent care clinic for your nasty cough, your provider will try to narrow down what type of illness you might have. They might do a COVID test, a strep test, or a flu test, to name a few. They might use symptoms and your history (like how long you’ve had the cough) to figure out what could be going on. And, yes, sometimes it turns out you have a bacterial illness. Or you may have had a virus to start with, but caught something bacterial in addition—because sometimes bacteria attack us when our immune systems are weak. Antibiotics would be appropriate if your illness is bacterial.

But an awful lot of the time, a virus is just a virus. And most of what we call “colds” are caused by viruses: rhinoviruses, adenoviruses, RSV, and the non-COVID coronaviruses are all common culprits when you have a cough and a sniffle. Your doc doesn’t even need to narrow down exactly which you have, because the treatment for mild cases is the same for all of them: rest, fluids, and time.

Not only will antibiotics not help a viral illness, they have downsides. One is that your body’s “good bacteria,” like gut flora, are affected by antibiotics. If you’ve ever had diarrhea when taking an antibiotic, you know what I mean. There are even life-threatening gastrointestinal infections like C. diff that can result from antibiotic use.

Another downside is that using antibiotics when they’re not needed can increase the spread of antibiotic-resistant bacteria, which is a growing problem in the world today. Part of the reason C. diff is so hard to treat is that it’s antibiotic-resistant. The same goes for MRSA, or methicillin-resistant Staphylococcus aureus. When we use antibiotics, we kill off bacteria that are not resistant, which can open up opportunities for resistant germs to spread.

And finally, there’s always the possibility of an adverse reaction to any drug. You may be allergic to an antibiotic, or you may experience side effects of the drug—which can range from mild annoyances like stomach cramps to potentially serious conditions. So it’s best not to take antibiotics if you don’t need them.

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