Why are antibiotics not effective against viruses?

Antibiotics don’t affect viruses because they only work on bacterial structures.

Antibiotics (also called antibacterials) are put into different classes according to their mechanisms. Some, like penicillin, work by interfering with the peptidoglycan that makes up bacterial cell walls. Others work by inhibiting the bacteria’s ability to synthesize essential parts of its structure and functionality – like proteins, DNA, or RNA.

However, viruses (which, unlike bacteria, are not alive) don’t have anything for the antibiotics to interfere with. Viruses on their own only consist of genetic material, a capsid protein that protects the genetic material, and sometimes a fatty lipid coat.

An antibiotic that interferes with the cell wall would be useless, because no virus has a peptidoglycan cell wall. An antibiotic that interferes with protein/DNA/RNA synthesis would be useless, because the viruses don’t have the machinery to synthesize anything – in order to reproduce, they use the machinery of the host cell they infect. Viruses simply have nothing that the antibiotics can target.

There are drugs that are called antivirals – these drugs only target viral structures, and would not be effective against antibacterials.