Antibiotics for Flu, Sore Throat or Cystitis? When They Work
A fever with aching bones, a scratchy throat or a burning sensation when you pee: in all three cases, people often ask for an antibiotic. But antibiotics don't work equally well in every case, and in one of them, taking one can harm you months later.
How an antibiotic works
An antibiotic targets parts that only bacteria have, such as their cell wall or their ribosomes. It works like a key made for one specific lock.
Viruses don't have that lock. They get inside your cells and use their machinery. That is why an antibiotic can't act against them.
Flu: antibiotics won't cure it
A high fever with aching bones is usually flu, which is caused by the influenza virus. An antibiotic won't cure it or shorten the fever.
What helps against flu is the annual vaccine and, for people at risk, an antiviral prescribed at the doctor's discretion. An antibiotic only makes sense if a bacterial complication develops, such as pneumonia.
Sore throat: test first
The throat is the in-between case. In adults, most cases of pharyngitis are viral. Only between 5% and 15% are caused by a bacterium, group A streptococcus. In children, that proportion is higher.
There are four clues that help tell them apart: fever, white patches on the tonsils, swollen glands in the neck and no cough. A cough points to a virus.
If the clues fit, a rapid swab test gives a result within minutes. If it comes back positive, an antibiotic is worth taking: it shortens the symptoms somewhat, reduces the spread of infection and prevents complications such as rheumatic fever.
Cystitis: here antibiotics do work
A burning sensation when you pee is usually due to cystitis. Around 80% of cases are caused by Escherichia coli, a bacterium from your own gut.
Once cystitis is confirmed, the antibiotic works. In women without complications, a single dose is sometimes enough.
What happens if you take an antibiotic you don't need
E. coli is living in your gut right now. If you take an antibiotic you don't need, for example for flu, the susceptible bacteria die. The resistant ones, the ones that have changed the lock, survive and take their place.
A review published in the BMJ in 2010 measured this. After a course of treatment, bacteria in the patient's urine and airways were more often resistant, especially during the first month. The difference could last up to a year.
Resistance isn't just a global problem: it takes hold in you. If you get cystitis during those months, the usual antibiotic is more likely to fail. There are also immediate costs: diarrhoea, allergies and, sometimes, Clostridioides difficile, a bacterium that thrives in the gap left by the others.
When to see a doctor straight away
Cystitis with fever, chills or pain in the lower back: the infection may have spread up to the kidney.
A sore throat that stops you opening your mouth or swallowing your own saliva. Flu with shortness of breath, or a fever that comes back just when you were starting to feel better.
Frequently asked questions
Does green mucus mean there's a bacterial infection?
No. The green colour comes from an enzyme in your white blood cells, which is also present when you have a cold.
Can I take leftover antibiotics from a previous illness?
It isn't recommended: they're usually an incomplete dose, possibly the wrong antibiotic, and they encourage another round of resistant bacteria. They belong in your pharmacy's medicine take-back bin.
Do antibiotics shorten a flu fever?
No. Flu is caused by a virus, and an antibiotic won't cure it or shorten the fever.
Does every sore throat need antibiotics?
No. In adults, most are viral; an antibiotic is only worth taking if a rapid test confirms group A streptococcus.
Informative article written with the help of AI tools from the verified script of the video. The video is in Spanish.