Why don't antibiotics work on viruses?

Because antibiotics attack structures and processes that bacteria have and viruses do not. It is not a matter of strength or dose — the targets simply are not present.

Bacteria are living cells. They have a cell wall, their own ribosomes for building proteins, their own machinery for copying DNA, and their own metabolism. Antibiotics exploit exactly these features. Penicillin and related drugs disrupt cell wall construction, so the bacterium bursts. Others jam bacterial ribosomes, which differ enough from human ribosomes to be targeted selectively. Others interfere with bacterial DNA replication enzymes.

A virus has none of this. It is essentially genetic material in a protein coat. It has no cell wall to disrupt, no ribosomes to jam, no metabolism to poison. It replicates by hijacking your cells' machinery — so a drug that stopped viral replication by attacking that machinery would be attacking your own cells too. This is precisely why antiviral drugs are harder to develop and tend to be specific to particular viruses, targeting the few proteins the virus does supply itself.

Why this matters practically: colds, flu, most sore throats, most coughs and most sinus infections are viral. Antibiotics do nothing for them. Taking them anyway carries real costs — side effects, disruption of your gut bacteria, and contribution to antibiotic resistance, which is among the most serious long-term threats in medicine.

"Finishing the course" is standard advice, though the evidence behind it is now debated and some guidance is moving toward shorter courses. Follow what your prescriber tells you rather than stopping early because you feel better, and never save leftovers for next time.

Bacterial complications can follow a viral illness — which is why symptoms that worsen after initially improving are worth a medical review.