Why antibiotics are powerless against common cold viruses

A cold, medically known as an upper respiratory tract infection, is triggered by over 200 different viruses, with rhinoviruses leading the pack. Acute bronchitis and most sore throats are also caused by viruses. An antibiotic simply cannot kill a virus because it lacks the targets that the medication exploits in bacteria.

The Austrian health portal puts it very clearly: "Antibiotics are not useful for a simple cold." [2] They won't shorten your runny nose or your cough because they miss the actual pathogen completely. What you are left with are the side effects, without any of the hoped-for benefits.

The persistent myth of green phlegm

"But my phlegm is yellow-green, so it must be bacterial!" This belief is very widespread, but it's actually incorrect. The green colour comes from an enzyme produced by immune cells (myeloperoxidase) that are actively fighting the infection. It shows that your immune system is hard at work, not that bacteria are causing mischief.

Studies clearly show that whether your phlegm is clear, yellow, or green tells us nothing about whether antibiotics will help you. [3] Colour alone is no reason for a prescription, even if it is still treated as such in many doctors' surgeries.

What the studies really show

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

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

Acute bronchitis: A Cochrane review of 17 studies with over 5,000 participants showed that antibiotics shorten a cough by less than half a day on average, out of an illness lasting eight to ten days. In return, one in 24 treated people experienced additional side effects. [3]

Waiting instead of prescribing straight away: If a prescription is given "just in case" and only filled if things don't get better, antibiotic use drops from 93 to 29 per cent, without patients feeling any worse for it. [5]

When antibiotics are actually needed for respiratory infections

Controversial doesn't mean "never". There are absolutely situations where antibiotics save lives! The key is to use them in a targeted way, following a medical assessment rather than just on suspicion.

These situations include a medically diagnosed bacterial pneumonia, tonsillitis caused by streptococcal bacteria (assessed by a doctor using tools like the Centor or FeverPAIN score) [6], or infections if you have a severely weakened immune system. You should seek medical help if you notice these warning signs: a high fever lasting several days, shortness of breath, severe pain on one side, bloody phlegm, or if you suddenly feel much worse after starting to recover.

The resistance problem: Why every unnecessary tablet matters

Every antibiotic you take puts pressure on bacteria to become resistant. This pressure affects pathogens just as much as the billions of friendly, useful germs in your body. Antibiotic resistance means that medicines that once worked reliably can fail when it really matters.

This is not an abstract future scenario. According to a major analysis in the medical journal The Lancet, bacterial resistance was directly responsible for around 1.27 million deaths worldwide in 2019. Over 39 million additional deaths are projected by 2050. [7] The World Health Organization lists antibiotic resistance as one of the greatest threats to global health. [8] Every avoidable prescription we save helps buy precious time for our future.