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Key points

  • Antibiotics can only treat infections caused by bacteria.
  • Antibiotics can interfere with some other medicines, including the contraceptive pill, making them less effective.
  • Always finish your course of antibiotics, even if you feel better.

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

Why would I take antibiotics?

You may be prescribed antibiotics to treat an infection that is caused by bacteria. Common infections that are caused by bacteria include staphylococcal wound infections, some types of food poisoning, such as salmonella, and certain sexually transmitted infections (STIs), for example chlamydia.

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 doctor won't prescribe them to treat these conditions. See our frequently asked questions for more information.

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 erythromycin. 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 a short course of two to four days or a standard course of seven to 10 days. Make sure you follow the advice given to you by your doctor, nurse prescriber or dentist and in the patient information leaflet that comes with your medicine. You will need to take your antibiotics at regular, specified time intervals, such as every four, six, eight or 12 hours. This ensures that there is always a steady level of the medicine in your body.

You may have to follow specific instructions about exactly when to take your antibiotics. Sometimes you may need to take your antibiotics on an empty stomach – usually an hour before meals or two hours afterwards so there is no food in it. There are other types of antibiotics that you need to take when you eat a meal.

You may need to be careful not to eat certain foods or drinks if you’re taking some types of antibiotics. Don’t drink any alcohol if you're taking the antibiotic metronidazole, as it can make you feel very sick. If you have been prescribed tetracycline antibiotics, don’t take them with milk or other dairy products as this can reduce how well they are absorbed. 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’re given, or you don’t finish the whole course, the bacteria may be able to return in a form that are resistant to the medicine. Always take your antibiotics as directed by your doctor and don't stop taking them early, even if your symptoms improve. See our frequently asked questions for more information.

Antibiotics usually come as tablets or capsules (or syrup for children and people who have difficulty swallowing). You may also be given them by injection, or you may apply them directly to the affected part of your body as drops, lotions or ointments.

Special care

If you have problems with your liver or kidneys, tell your doctor, nurse prescriber or dentist before he or she prescribes you antibiotics. You should also tell your 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.

It’s possible that you may 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. If you develop a fungal infection, you may then need to take an antifungal medicine to treat it.

More serious side-effects of antibiotics include kidney problems, blood disorders, increased sensitivity to the sun and deafness. However, these are rare and often specific to the type of antibiotic you’re taking.

If your antibiotics make you feel ill or you want to stop taking them for any reason, contact your doctor as soon as possible. Depending on your infection, he or she may be able to offer you an alternative antibiotic, although this won’t always be possible.


Some people are allergic to antibiotics, particularly penicillin and cephalosporins. If you’re allergic to an antibiotic, you may get an itchy rash and possibly some mild wheezing. If you have a severe allergic reaction, this is called anaphylaxis. The symptoms include swelling of your face, throat and tongue, and having difficulty breathing. Your blood pressure may fall and you may lose consciousness. Anaphylaxis can be serious or even fatal. Seek urgent medical attention if you think you’re having these symptoms after taking antibiotics or any other medicines.

Always tell your doctor, nurse prescriber or dentist 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 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’re taking antibiotics that interfere with the contraceptive pill or have vomiting or diarrhoea when you're taking antibiotics, it’s important that you use additional, barrier contraception such as a condom.

Antibiotics can interact with a number of other medicines and herbal remedies. It's important to check with whoever prescribes your antibiotics before you take anything else at the same time.


Some infections are resistant to certain antibiotics. This means that the antibiotic isn’t effective at getting rid of the infection. For example, if you become infected with meticillin-resistant Staphylococcus aureus (MRSA), it won’t be possible to treat you 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 to resist the treatment. The few bacteria that survive can then reproduce, causing an infection that is resistant to that particular antibiotic. It’s very important that you complete a course of antibiotics, even if you feel better, as this is the best way to prevent any bacteria from surviving.

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, nurse prescriber or dentist 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.


Generic names Example of common brand names
amoxicillin Amoxil
ampicillin Penbritin
benzylpenicillin (penicillin G) Crystapen
co-amoxiclav Augmentin
flucloxacillin Floxapen 
phenoxymethylpenicillin (Penicillin V)  
clarithromycin Klaricid
erythromycin Erymax, Erythrocin, Erythroped
cefaclor Distaclor
cefalexin Ceporex, Keflex
doxycycline Vibramycin-D
oxytetracycline Oxymycin
gentamicin Cidomycin, Genticin
neomycin Nivemycin
ciprofloxacin Ciloxan, Ciproxin
chloramphenicol Kemicetine, Minims


metronidazole Flagyl, Metrolyl, Metrosa
rifampicin Rifadin, Rimactane
trimethoprim Trimopan
vancomycin Vancocin


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

For sources and links to further information, see Resources.


Produced by Polly Kerr, Bupa Health Information Team, March 2013.

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  • This information was published by Bupa's Health Information Team and is based on reputable sources of medical evidence. It has been reviewed by appropriate medical or clinical professionals. Photos are only for illustrative purposes and do not reflect every presentation of a condition. The content is intended only for general information and does not replace the need for personal advice from a qualified health professional. For more details on how we produce our content and its sources, visit the about our health information page.

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