How antibiotics work and the main classes

What antibiotics do — and do not do

Antibiotics work only against bacteria, the tiny living organisms behind infections such as many chest, urine, skin and wound infections. They do this either by killing bacteria outright or by stopping them multiplying so the body's own immune system can clear the infection. Crucially, antibiotics have no effect on viruses, which cause most coughs, colds, sore throats, flu and many other everyday illnesses. Taking an antibiotic for a viral infection will not help you recover faster and needlessly exposes you to side effects and resistance. This is why clinicians assess whether an infection is likely bacterial before prescribing, and why many minor infections are best managed with rest, fluids and time.

How they target bacteria

Antibiotics are clever because they attack features that bacteria have but our own cells do not, allowing them to harm the bacteria while largely sparing us. Some, such as penicillins, break down the bacterial cell wall, a protective outer layer human cells lack, causing the bacterium to burst. Others interfere with the bacterium's ability to make the proteins it needs to survive and grow. Some block the copying of bacterial genetic material, and others disrupt the chemical processes bacteria use to build essential molecules. Different antibiotics are described as narrow-spectrum, hitting a limited range of bacteria, or broad-spectrum, affecting many types — and narrow-spectrum choices are preferred when possible to limit collateral damage.

The main classes at a glance

There are several major antibiotic families. Penicillins and cephalosporins (together called beta-lactams) attack the cell wall and are widely used first-line choices. Macrolides offer an alternative, often for people with penicillin allergy or certain chest infections. Tetracyclines are used for infections such as acne and some respiratory and skin infections. Quinolones are powerful but reserved for specific situations because of particular side effects. Aminoglycosides are strong hospital antibiotics for serious infections. Others include nitrofurantoin, commonly used for urine infections, and metronidazole for certain gut and dental infections. Clinicians choose based on the likely bacteria, where the infection is, allergies and local resistance patterns — not on strength alone.

Using antibiotics safely

Antibiotics are generally safe, but like all medicines they can cause side effects, most commonly upset stomach, diarrhoea or thrush, and occasionally allergic reactions. Anyone who has had a serious reaction, such as swelling or difficulty breathing, must tell their clinician, as this shapes which antibiotics are safe. Always take the antibiotic exactly as prescribed and complete the course as directed by your prescriber, rather than stopping early or saving leftovers. Never take antibiotics left over from before, share them with others, or press for them when a clinician advises they are not needed. Some antibiotics interact with other medicines or alcohol, and a pharmacist can advise on how and when to take yours and what to watch for.

Antibiotic resistance and why it matters

Antibiotic resistance is one of the biggest threats to global health. Every time antibiotics are used, bacteria have a chance to adapt and survive, and overuse or misuse speeds this up, producing infections that are harder — sometimes impossible — to treat. This affects everyone, because resistant bacteria can spread between people. Protecting antibiotics is a shared responsibility. You can help by only using antibiotics when genuinely needed, following instructions, never demanding them for viral illnesses, and preventing infections in the first place through good hygiene and staying up to date with vaccinations. Clinicians support this through antimicrobial stewardship — prescribing the right antibiotic only when necessary — so these vital medicines keep working for future generations.