What are antibiotics?

Antibiotics are a group of drugs used to treat various infections caused by bacteria. The first antibiotic was discovered by Sir Alexander Fleming in 1928 in a significant breakthrough for medical science. The development of antibiotics is probably the largest advance in medicine in the 20th century, and has saved millions of lives worldwide from infections such as TB.

Since Fleming's original development of penicillin, a wide range of antibiotics has been developed. Originally produced from moulds and fungi, many antibiotics are now produced synthetically.

Antibiotics are not effective against infections caused by viruses, so if you go to your doctor with a viral illness such as a cold, flu or a viral sore throat, antibiotics have little or no role to play.

Some antibiotics are effective only against specific bacteria. For example, if you have a streptococcal throat, penicillin may be prescribed. Others are effective against a wide range of bacteria. These are called broad spectrum antibiotics. This type of antibiotic is often used to treat respiratory or urinary tract infections because it is difficult to establish what specific bacteria are causing the condition.

Antibiotics can be classified into five main categories:

PENICILLINS: eg. Amoxycillin, Amoxil, Augmentin, Calvepen, Clonamox, Floxapen

CEPHALOSPORINS: eg. Cefodox, Distaclor, Keflex, Suprax, Zinnat


TETRACYCLINES: eg. Clinimycin, Hostacycline, Minocin, Vibramycin

OTHERS: eg. Ciproxin, Erymax, Erythrocin, Flagyl, Ipral, Klacid, Zithromax

The choice of antibiotic depends on the type of bacteria, the site of infection, the general health of the individual and any history of allergic reaction to antibiotics.

Antibiotics may be taken as tablets, capsules, liquids, creams or in intravenous infusions (directly into the veins through a drip). Doctors recommend an antibiotic based on where the infection is, the type of infection, the general health of the individual and the antibiotic being used.

How do antibiotics work?

There are many different antibiotics and they work in different ways. Essentially, antibiotic drugs either kill bacteria (these are known as bactericidal) or stop bacterial growth, allowing the body’s immune system to deal with the infection (these are known as bacteriostatic).

The most commonly used antibiotics, penicillins and cephalosporins, are bactericidal and work by breaking down the cell walls of bacteria.

What are the dangers of inappropriate use?

Bacteria are capable of developing resistance to a previously effective antibiotic drug. This means that the specific antibiotic will no longer be of use in treating that infection. While resistance may develop in any case, a number of factors may speed up the development of resistance. These include:

• Widespread use of antibiotics to treat relatively minor infections in humans

• Widespread use of antibiotics in the rearing of animals which provide meat which is eaten by humans

• Failure to complete a course of antibiotics as prescribed by your doctor.

The danger of widespread resistance to antibiotics is that bacterial infections will become untreatable and resistant strains of bacteria will develop which will not respond to antibiotic treatment (so-called 'superbugs'). This will have serious consequences, initially for people with underlying medical conditions such as chronic lung disease or diseases which suppress the immune system, such as HIV.

There is huge concern among doctors that eventually these 'superbugs' could cause untreatable infections in the otherwise healthy population.

There is considerable concern in Ireland at present about the emergence of a type of infection which is resistant to methicillin, one of the most powerful antibiotics available. The infection, known as MRSA, thrives in hospitals, where people are in close proximity and may have their defences weakened by illness.

MRSA is a growing problem worldwide. One of the issues being stressed to health workers in hospitals is the importance of thorough and frequent hand washing to avoid spreading the disease from patient to patient. In Dublin, St James's Hospital was forced to close its Bone Marrow Transplant Unit in September 2000 following an outbreak of Vancomycin-Resistant Enterococcus (VRE), another dangerous antibiotic-resistant bacteria.

What are the possible side effects of antibiotics?

Antibiotic drugs can literally save lives and are effective in treating illnesses caused by bacterial infections when appropriately used. However, like all drugs, they have the potential to cause undesirable side effects. Many of these side effects are not dangerous, although they can make life miserable while the drug is being taken.

Most antibiotic drugs have the potential to cause nausea, vomiting or diarrhoea because they alter the normal balance of beneficial or 'friendly' bacteria that live in the gut. Alteration of this balance between certain types of bacteria and fungi in the body may also cause proliferation of particular yeasts in the body, causing thrush in the mouth, gut or vagina. Antibiotics may also cause skin rashes.

Other, less common, side effects may be caused by particular antibiotics. Your doctor will warn you of these if you need to take these antibiotics.

If you develop side effects, it is not wise just to stop taking the antibiotic. You should consult your doctor to discuss whether specific treatment of the side effects are needed or whether you should be changed to another antibiotic to treat the initial infection.

How do I know if I am allergic to an antibiotic?

Some people may be allergic to a particular class of antibiotics. If this is the case, these antibiotics should be avoided in the future and other antibiotics, from different categories, should be used to treat bacterial infections.

Allergic reactions cause swelling of the face, itching and a skin rash and, in severe cases, breathing difficulties. Allergic reactions require prompt treatment. This will include stopping the antibiotic and possibly taking anti-histamine drugs. Hospital admission may be necessary in severe cases.

Many people may believe they are allergic to an antibiotic if they develop vomiting, diarrhoea or a skin rash when taking an antibiotic. These symptoms are not caused by an allergy to the antibiotic, but are side effects of the drug. Side effects occur more commonly than allergic reactions.

The distinction is important because of its consequences for future treatment. If you develop side effects, then you should avoid that particular antibiotic in the future but you may be able to tolerate other antibiotics from the same class eg. if you develop side effects when taking one penicillin, you may well tolerate other penicillins in the future. However, if you are allergic to a particular penicillin you should avoid all penicillins (and also cephalosporins).

What special precautions are needed in taking antibiotics?

Your doctor will be aware of the special precautions in prescribing antibiotics if you have chronic liver of kidney disease. It is important to tell your doctor if you have a history of allergy to an antibiotic or have experienced side effects from an antibiotic. Some antibiotics are best avoided by women who are pregnant or breast feeding. You should inform your doctor of all other medications you are taking at the time as antibiotics can interact with other medications, particularly the oral contraceptive pill.

How long should a course of antibiotics last?

The choice of antibiotic, the dose and the length of the course will depend on the infection being treated and the general health of the person. Your doctor will prescribe the appropriate course for the particular illness. It is important to complete the course as prescribed to ensure that the illness is treated properly. Failure to complete antibiotic courses is also a contributory factor to antibiotic resistance.

Courses may be as short as a single dose of an antibiotic. Three or five day courses of antibiotics are commonly recommended for urinary tract infections. Bacterial infections of the throat or lungs may require between five and 10 days' treatment. Long-term antibiotics (for months) may be prescribed in the treatment of acne or the prevention of recurrent urinary tract infections.

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