Why antibiotics are powerless against common cold viruses

A cold, medically termed an upper respiratory tract infection, is caused by over 200 different viruses, predominantly rhinoviruses. Acute bronchitis and most cases of pharyngitis are also viral. An antibiotic cannot kill a virus because it simply lacks the target structures that it utilises in bacteria.

The Austrian Public Health Portal states unequivocally: "For a simple cold, antibiotics are not useful." [2] They shorten neither the rhinitis nor the cough, because they miss the pathogen. What remains are the side effects, without the hoped-for benefit.

The persistent myth of green mucus

"But the sputum is yellow-green, surely that must be bacterial!" This belief is widespread and yet incorrect. The green colour comes from an enzyme produced by immune cells (myeloperoxidase) that fight the infection. It indicates that your immune system is working, not that bacteria are at work.

Studies clearly show: whether sputum is clear, yellow, or green says nothing about whether antibiotics will help. [3] The colour alone is not a reason for a prescription, even though it is still treated as such in many general practices.

What the studies actually show

The evidence is clear, and it comes from the highest-quality medical sources: the Cochrane systematic reviews, which systematically synthesise all available studies.

Common cold: A Cochrane analysis found no benefit of antibiotics for the common cold. In adults, however, the risk of side effects under antibiotics was more than twice as high (factor of 2.6). [4]

Acute bronchitis: A Cochrane review of 17 studies involving over 5,000 participants showed that antibiotics shorten the cough by an average of less than half a day, with a disease duration of eight to ten days. In return, one in 24 treated patients suffered additional side effects. [3]

Delayed prescribing: If a prescription is given "just in case" and only redeemed if there is no improvement, antibiotic consumption drops from 93% to 29%, without any deterioration in patient outcomes. [5]

When antibiotics are actually necessary for respiratory tract infections

Controversial does not mean: never. There are situations in which antibiotics save lives. The key is to use them in a targeted manner, following a medical assessment rather than on suspicion.

These include clinically diagnosed bacterial pneumonia, streptococcal tonsillitis (medically assessed, for example using the Centor or FeverPAIN score) [6], or infections in severely immunocompromised patients. You should seek medical attention if you experience these red flag symptoms: high fever lasting several days, dyspnoea, severe unilateral pain, haemoptysis, or a condition that suddenly deteriorates after initial improvement.

The resistance problem: why every unnecessary tablet counts

Every antibiotic you take exerts pressure on bacteria to become resistant. This pressure affects pathogens as well as the billions of beneficial microbes in your body. Antimicrobial resistance means: medications that once worked reliably fail in an emergency.

This is not an abstract future scenario. According to a major analysis in the medical journal The Lancet, bacterial resistance was directly responsible for approximately 1.27 million deaths worldwide in 2019. More than 39 million additional deaths are projected by 2050. [7] The World Health Organization considers antimicrobial resistance to be one of the greatest threats to global health. [8] Every avoidable prescription buys a little more time against this future.