In short
Antibiotics do nothing against the viruses behind most coughs, colds and sore throats. Knowing which infections actually need them — and how to take a course properly — protects you from side effects and keeps the drugs working.
Emergency if: Swelling of the face, lips or throat, difficulty breathing or a widespread rash after a dose — this may be a severe allergic reaction. Also treat confusion, fast breathing or mottled skin with an infection as possible sepsis.
On your report
CRP, Procalcitonin, WBC / Total Leukocyte Count, Absolute Neutrophil Count
On this page
01
The Line Between Bacteria and Viruses
Antibiotics work by attacking structures and processes that bacteria have and human cells do not — a cell wall, a particular ribosome, a specific enzyme. Viruses are built differently and replicate inside your own cells, so there is simply nothing there for an antibiotic to act on.
That matters because most of the illnesses people ask for antibiotics for are viral: ordinary colds, influenza, COVID-19, the large majority of sore throats, and most acute coughs and chest colds. These settle on their own, though the timeline is longer than people expect — a cough can rumble on for three weeks after everything else has cleared.
02
When They Genuinely Are Needed
Bacterial pneumonia, most urinary infections, cellulitis and other skin infections, dental abscesses, confirmed strep throat, and sepsis all need antibiotics — the last of these urgently, within the first hour. Some ear and sinus infections need them too, usually when they are severe or have failed to settle.
Doctors also prescribe them preventively in specific situations, such as around certain operations or for people whose immune system is suppressed. In borderline cases a delayed prescription is sometimes used: you hold it and only start if things are not improving by an agreed point.
03
The Cost of Taking Them When You Don't Need Them
Antibiotics are not neutral. Diarrhoea, nausea and thrush are common; rashes and allergic reactions happen; and they disturb the gut's normal bacteria. That disturbance occasionally allows an infection called Clostridioides difficile to take hold, which causes severe, watery diarrhoea and can be serious, particularly in older adults.
They also interact with other medicines — a point that catches people out with blood thinners, where an antibiotic course can shift the INR and require extra monitoring.
04
Resistance, and a Common Misunderstanding
Every course applies evolutionary pressure: susceptible bacteria die, and any that can withstand the drug survive and multiply. Over time this produces infections that no longer respond to the usual first-choice tablets and need stronger or intravenous alternatives. The World Health Organization ranks antimicrobial resistance among the greatest threats to global health.
The common misunderstanding is that a person's body becomes resistant. It does not — the bacteria do, and resistant strains then circulate between people. That is why one person's unnecessary course is genuinely everyone's problem, and why the decision not to prescribe is often the more careful one.
05
Taking a Course Well
Take it exactly as prescribed, at the intervals given — the length has been chosen for that infection, and several conditions are now deliberately treated with shorter courses than they once were. Do not stop early because you feel better, and do not stretch a course out; if you are markedly better within a couple of days, ask the prescriber rather than deciding alone.
Never use leftover antibiotics or someone else's, since the drug and dose are matched to the organism and to you. And if you carry a penicillin-allergy label from childhood that has never been tested, ask about having it reviewed — many such labels turn out to be wrong, and they push people towards broader, less effective alternatives.
When to act
Emergency — act now
Swelling of the face, lips or throat, difficulty breathing or a widespread rash after a dose — this may be a severe allergic reaction. Also treat confusion, fast breathing or mottled skin with an infection as possible sepsis.
Same-day — call promptly
Getting worse, or no better, two to three days into a course; or severe watery diarrhoea during or in the weeks after one.
Routine — see a doctor
Repeated infections needing repeated courses, or an untested penicillin-allergy label — both are worth a proper review.
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