What it means
An antibiotic kills bacteria or prevents them multiplying. It has no effect whatsoever on viruses.
This single fact explains most of what is confusing about antibiotic advice. Colds, flu, most sore throats, most coughs, most sinus congestion and most ear infections in children are viral — an antibiotic cannot shorten them, and can only cause side effects.
Broad-spectrum antibiotics act against many bacteria; narrow-spectrum against few. Narrow is preferred where the organism is known, because broad-spectrum agents also destroy harmless gut bacteria and drive resistance.
In practice
Antibiotic choice is guided by the likely organism, local resistance patterns and culture and sensitivity results where available.
Antimicrobial resistance is a genuine and growing problem, and it is driven by cumulative unnecessary prescribing rather than by any single course.
Why it matters
Being refused an antibiotic is not being fobbed off. It is frequently the more difficult conversation and the correct decision.
Current UK guidance is to complete the course as prescribed, and to return if symptoms worsen or fail to improve. Leftover antibiotics should never be saved or shared — the dose, duration and choice are specific to one infection in one person.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026