Antibiotics target bacteria, not cold viruses
A winter cold is usually caused by a virus. Antibiotics work against bacteria: depending on the drug, they may kill bacteria or stop them from multiplying. Some do this by disrupting bacterial cell walls, which human cells do not have; others interfere with bacterial processes needed to make proteins or copy genetic material.
Cold viruses work differently. They enter the body’s cells and use those cells to make more virus. Antibiotics do not block that process, so they do not shorten an ordinary viral cold or relieve its symptoms. A runny nose, cough, or sore throat can linger while the immune system clears the infection, even when no antibiotic is needed.
When they can help during a respiratory illness
Sometimes a bacterial infection develops alongside or after a viral illness, or symptoms turn out to come from a bacterial condition rather than a cold. In those cases, a clinician may consider an antibiotic based on the symptoms, examination, and, when useful, testing. The drug is chosen for the suspected bacteria and infection; antibiotics are not interchangeable remedies for feeling sick.
Taking one when it cannot help still has costs. It can cause side effects, including diarrhea or allergic reactions, and it exposes bacteria to a drug that can favor the survival of resistant strains. Those bacteria can make future infections harder to treat.
An antibiotic can treat a susceptible bacterial infection; it cannot treat the virus behind a typical cold. That distinction is why the same cough or congestion does not, by itself, show that an antibiotic will help.
