A group of drugs used primarily to treat infections caused by bacteria

Common drugs


  • Amoxicillin

  • Ampicillin

  • Co-amoxiclav

  • Flucloxacillin

  • Phenoxymethylpenicillin


  • Cefaclor

  • Cefalexin

  • Cefixime

  • Cefradine

  • Ceftazidime

  • Cefuroxime


  • Azithromycin

  • Clarithromycin

  • Erythromycin


  • Doxycycline

  • Lymecycline

  • Minocycline

  • Oxytetracycline

  • Tetracycline


  • Amikacin

  • Gentamicin

  • Neomycin

  • Streptomycin

  • Tobramycin


  • Co-trimoxazole

  • Sulfadiazine


  • Ciprofloxacin

  • Levofloxacin

  • Norfloxacin

  • Ofloxacin

    Other antibiotics

  • Chloramphenicol

  • Clindamycin

  • Fusidic acid

  • Metronidazole

  • Trimethoprim

  • Vancomycin

Antibiotics are among the most commonly prescribed drugs in the world. They are used to treat and prevent bacterial infections. The drugs work either by killing bacteria directly or by halting their multiplication so that the body’s immune system is able to overcome the remaining infection.

Antibiotics are classified in groups, such as penicillins, cephalosporins, and tetracyclines, according to their chemical composition and the way in which they work. One of the most commonly used groups of antibiotics is penicillins (see How penicillins work). Some antibiotics work against a wide range of bacteria and are called broad-spectrum antibiotics. Other antibiotics work against only one or two types of bacteria and are known as narrow-spectrum antibiotics. Some also work against other types of infection, such as those caused by certain protozoa (see Antiprotozal drugs).

Why are they used?

Antibiotics are most commonly used for the short-term treatment of minor infections, such as infections of the ear, throat, or urinary tract. The drugs may also be used in the treatment of serious infections such as septicaemia and bacterial meningitis.

On occasion, long-term treatment with low-dose antibiotics may be given to prevent infection in people whose immunity is reduced, such as those with HIV infection or AIDS and people who are taking immunosuppressants. Long-term use of antibiotics may also be prescribed for other conditions, such as acne.

How are they used?

Most bacterial infections are treated with oral antibiotics. Eye and ear infections are often treated using antibiotic drops (see Drugs for eye and ear disorders). To treat severe infections, when high doses of antibiotics are needed immediately, the drugs may be given by intramuscular injection or by intravenous injection or infusion.

If you have a bacterial infection, your doctor will probably prescribe a specific antibiotic that is effective against the bacterium which is most likely to be causing that infection. For certain conditions, such as pneumonia or a wound infection, your doctor may carry out tests to identify the specific infecting organism before prescribing a narrow-spectrum antibiotic. In the meantime, you will probably be treated with a broad-spectrum antibiotic. More than one antibiotic may be prescribed to reduce the risk of antibiotic resistance developing.

When you are taking antibiotics, you should continue treatment even after your symptoms improve. Finish the entire course prescribed by your doctor in order to eradicate the infection. Bacterial resistance to antibiotics is increasing, due in part to incorrect use.

Antibiotics are available by prescription only. The choice of antibiotic may be restricted if you are pregnant, have a history of drug allergy, have impaired kidney or liver function, or if you are taking other drugs that might interact with a particular antibiotic.

What are the side effects?

Antibiotics do not usually cause serious side effects. The most common is diarrhoea, although it is not usually severe. Antibiotics can alter the balance between normal bacteria and the yeast Candida albicans that occurs naturally in or on the body; this imbalance may lead to an overgrowth of the yeast (see Candidiasis). In some cases, antibiotics destroy harmless bacteria in the bowel that prevent the growth of disease-causing organisms. Rarely, this results in the potentially serious condition pseudomembranous colitis, which can cause diarrhoea and severe dehydration.

Penicillins may cause rashes and in some people a rare, life-threatening allergic reaction known as anaphylaxis. This is more common when the drugs are given by injection and occurs mainly in people who have previously had a mild allergic reaction to a penicillin. If you are allergic to one type of penicillin, you will be allergic to others in the same group. About 1 in 10 people who is allergic to penicillin is also allergic to cephalosporins. If you have an adverse reaction, such as a rash, stop taking the drug and see your doctor promptly. If you have previously had a reaction, tell your doctor.

Tetracyclines can cause changes to growing bones and teeth and are therefore not prescribed for young children or pregnant women. In some rare cases, aminoglycosides can damage the kidneys, cause severe skin rashes, and affect the sense of hearing and balance.

Quinolones occasionally cause convulsions and should not be taken by people with epilepsy. They may rarely cause tendon damage and, in children, damage to joints.

Sulphonamides sometimes cause serious side effects, such as rashes and kidney damage.

Drug Action: How Penicillins Work

From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.

The subjects, conditions and treatments covered in this encyclopaedia are for information only and may not be covered by your insurance product should you make a claim.

Back to top