Antibiotics

Published by Bupa's Health Information Team, May 2011.

This factsheet is for anyone taking antibiotics, or who would like information about them.

Antibiotics are used to treat infections caused by bacteria. In the UK, antibiotics are only available on prescription from a doctor, a nurse prescriber or a dentist.

Why would I take antibiotics?

You may be prescribed an antibiotic to treat an infection that is caused by bacteria.

Common infections that may be caused by bacteria include staphylococcal wound infections, campylobacter (food poisoning), and chlamydia (a sexually transmitted disease).

Many infectious illnesses are caused by viruses rather than bacteria. These include influenza (flu), measles, mumps and hepatitis. Antibiotics don’t work against viruses, so your GP won't prescribe them to treat these conditions.

If you're having an operation, you may be given antibiotics to prevent an infection.

What are the main types of antibiotics?

Antibiotics may be classed as broad-spectrum, which means they can get rid of infections caused by a wide range of different bacteria. Examples of these include amoxicillin and cefotaxime. Other antibiotics only work against specific types of bacteria and are known as narrow-spectrum antibiotics. Examples of these include vancomycin and teicoplanin.

There are at least 70 different antibiotics that are grouped into classes depending on their chemical structure. Classes of antibiotics include penicillins, tetracyclines, macrolides and cephalosporins.

How do antibiotics work?

Some antibiotics work by killing bacteria. They usually do this by interrupting the chemical processes that the bacteria use to make their cell walls. Penicillin is an example of an antibiotic that works in this way.

Other antibiotics work by stopping bacteria from growing and multiplying. Most bacteria don't live long, so the infecting bacteria will eventually die out once you have started treatment with these medicines. Chloramphenicol is an example of this type of antibiotic.

How to take antibiotics

You will be prescribed antibiotics to take for a specific period of time. This can be three days or seven to 10 days. Make sure you follow the advice given to you by your doctor, dentist or nurse and in the patient information leaflet. You will need to take them at regular, specified time intervals, such as every four, six, eight or 12 hours.

You may have to follow specific instructions about exactly when to take your antibiotics. Sometimes you may need to take your antibiotics when there is no food in your stomach – usually an hour before meals or two hours afterwards. Other antibiotics should be taken during meals.

Some antibiotics should not be taken with certain foods and drinks. You shouldn't drink any alcohol if you're taking the antibiotic metronidazole, as it can make you feel very sick. If your doctor has prescribed tetracycline antibiotics, you shouldn’t take them with milk or other dairy products as this can reduce their absorption into your bloodstream. Make sure you follow the instructions on the medicine label or patient information leaflet, and if you have any questions, ask your doctor or pharmacist for advice.

If you don't take your antibiotics according to the instructions you are given, or finish the whole course, the bacteria may become resistant to them. Always take your antibiotics as directed by your doctor and don't stop taking them early, even if your symptoms improve.

Antibiotics usually come as tablets or capsules (or syrup for children). They can also be given by injection, or applied directly to the affected part of your body such as your skin, eyes or ears as drops, lotions or ointments.

Special care

If you have problems with your liver or kidneys, tell your doctor, dentist or nurse before they prescribe you antibiotics.

You should also tell your doctor, pharmacist or other health professional if you're pregnant or breastfeeding.

Side-effects of antibiotics

Side-effects are the unwanted effects of taking a medicine. The most common side-effects of antibiotics are diarrhoea, feeling sick and vomiting but you may not have any obvious side-effects.

Some people get a fungal infection such as thrush after treatment with broad-spectrum antibiotics. This is because as well as killing the harmful bacteria causing your infection, antibiotics may also destroy your body's harmless bacteria that stop micro-organisms such as fungi growing out of control. This side-effect is more common if you have been taking antibiotics for an extended period of time.

More serious side-effects of antibiotics include kidney problems, blood disorders, increased sensitivity to the sun and deafness. However, these are rare.

If your antibiotics make you feel ill or you want to stop taking them for some reason, contact your doctor as soon as possible. He or she may be able to offer you an alternative antibiotic, although this may not be possible for some infections.

Allergies

Some people are allergic to antibiotics, particularly penicillin and similar medicines such as cephalosporins. If you are allergic to an antibiotic, you may get a rash. If your face and tongue swell up and you have difficulty breathing, this is called an anaphylactic reaction and it can be serious or even fatal. Seek urgent medical attention if you think you are having these symptoms after taking antibiotics or any other medication.

Always tell your doctor or pharmacist if you think you have had an allergic reaction to an antibiotic before. As an alternative to penicillin, you may be prescribed erythromycin, which works against the same types of bacteria.

Interactions of antibiotics with other medicines

Certain antibiotics (for example penicillin and rifampicin) can stop the contraceptive pill from working. Any diarrhoea or vomiting that you get when taking an antibiotic may also stop your contraceptive pill from being absorbed into your body. If you are taking antibiotics that interfere with the contraceptive pill or have vomiting and diarrhoea when you're taking antibiotics, you should use additional (ie barrier, such as a condom) contraception.

There are a number of important interactions between antibiotics and other medicines or herbal remedies, so it's important to check with your doctor or pharmacist before you take any other medicines or herbal remedies at the same time as antibiotics.

Resistance

Some infections are resistant to certain antibiotics. For example, infections with methicillin-resistant Staphylococcus aureus (MRSA) bacteria can't be treated with standard antibiotics.

Resistance can start when the bacteria causing an infection aren't completely killed off by an antibiotic. Some bacteria survive because they have a genetic mutation that helps them resist the treatment. The few bacteria that survive can then reproduce, causing an infection that is resistant to that particular antibiotic.

If you complete a course of antibiotics and don’t get better, it could be because the bacteria causing your infection are resistant to them. If this happens, your doctor, dentist or nurse prescriber will usually prescribe a different antibiotic for you to take.

Names of common antibiotics

Examples of common antibiotics are shown in the table below. The antibiotics are grouped by class.

You may have noticed that your medicine has two or more names. All medicines have a generic name. Many medicines also have one or more brand name. Generic names are in lower case, whereas brand names start with a capital letter.

For answers to frequently asked questions on this topic, see Common questions.

For sources and links to further information, see Resources.