Antibiotics Do Not Work on Colds — Here Is Why
They kill bacteria. A cold is a virus. Taking them anyway gives you the side effects without any of the benefit, and contributes to a problem that is already killing people.
What to remember
- Antibiotics target bacterial structures that viruses do not have.
- Green mucus indicates immune activity, not bacterial infection.
- Antimicrobial resistance is associated with very large numbers of deaths globally each year.
- Delayed prescriptions are a useful middle ground where a GP is uncertain.
Different machinery entirely
Antibiotics work by attacking structures and processes specific to bacteria: cell wall synthesis, bacterial ribosomes, bacterial DNA replication. Viruses have none of these. They are not cells at all; they hijack your own cells to reproduce. There is no target for the drug to act on, which is why the failure is absolute rather than partial. Colds, flu, most sore throats, most coughs and most sinus infections are viral.
The green mucus myth
Mucus turns yellow or green because neutrophils, a type of white blood cell, contain a green-tinged enzyme and arrive in numbers during any infection. The colour tells you your immune system is working, not what it is working against. Studies of patients presenting with discoloured nasal discharge find antibiotics make little difference to recovery. It is one of the most common reasons people request them, and one of the least justified.
What the overuse costs
Bacteria exposed repeatedly to antibiotics evolve resistance, and resistant infections are harder and sometimes impossible to treat. Global analyses attribute well over a million deaths a year directly to antimicrobial resistance, with several million more associated. This is not an abstract future problem. Every unnecessary course contributes, and the individual taking it also risks diarrhoea, thrush, allergic reactions and C. difficile infection for no benefit at all.
What to ask for instead
Ask what would change the picture and when you should come back — a clear safety net is more useful than a prescription. Where a GP is genuinely uncertain, a delayed or back-up prescription, to be collected only if things have not improved by a stated point, reduces antibiotic use substantially without worsening outcomes. It is a recognised approach in NICE guidance, not a fob-off.
The numbers, in one place
| Measure | Figure |
|---|---|
| What antibiotics target | bacterial cell walls, ribosomes and DNA replication |
| Why they fail on viruses | viruses have none of those structures |
| Cause of green mucus | an enzyme in white blood cells, not bacteria |
| Typical cold duration | 7-10 days, cough sometimes 3 weeks |
| Deaths directly attributed to AMR globally | over a million a year |
| Useful middle ground | a delayed or back-up prescription |
| Common harms of unnecessary courses | diarrhoea, thrush, allergy, C. difficile |
Do
- Ask what symptoms should prompt you to come back, and by when
- Finish a course exactly as prescribed when antibiotics are genuinely indicated
- Return unused antibiotics to a pharmacy
- Accept a delayed prescription where it is offered; it is an evidence-based approach
Do not
- Do not press for antibiotics because mucus has turned green
- Do not take antibiotics left over from a previous illness, or someone else's
- Do not assume a longer illness automatically means a bacterial one
When to speak to a doctor
Contact a GP or NHS 111 for a cough lasting more than three weeks, fever that persists beyond five days, symptoms that improve and then get suddenly worse, difficulty swallowing or opening your mouth with a sore throat, or if you are immunosuppressed. Call 999 for difficulty breathing, chest pain, confusion, or a rash that does not fade when pressed with a glass.
Where to go next
- What Actually Supports Your Immune System
- Vitamin D in Winter: Who Needs a Supplement
- Cold or Flu: Telling Them Apart
Where this comes from
- NICE - Antimicrobial stewardship and respiratory tract infection guidance
- NHS - Antibiotics
- UK Health Security Agency - antimicrobial resistance
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.
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