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.
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.
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.
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 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.
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.
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 name Example of common brand names Penicillins amoxicillin Amoxil ampicillin Penbritin benzylpenicillin (penicillin G) Crystapen co-amoxiclav Augmentin flucloxacillin Floxapen phenoxymethylpenicillin (Penicillin V) Macrolides clarithromycin Klaricid erythromycin Erymax, Erythrocin, Erythroped Cephalosporins cefaclor Distaclor cefalexin Ceporex, Keflex cefotaxime Tetracyclines doxycycline Vibramycin-D oxytetracycline Oxymycin tetracycline Aminoglycosides gentamicin Cidomycin, Genticin neomycin Nivemycin Quinolones ciprofloxacin Ciloxan, Ciproxin Others chloramphenicol Kemicetine, Minims
metronidazole Flagyl, Metrolyl, Metrosa rifampicin Rifadin, Rimactane trimethoprim Trimopan vancomycin Vancocin
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.
How does my doctor choose the right antibiotic to treat my infection?
Your doctor (or nurse prescriber or dentist) will prescribe you an antibiotic based on your symptoms and your medical history. This will take into account whether you’re allergic to any antibiotics, if you’re pregnant and the severity of your infection. Some bacterial infections will get better on their own so your doctor may not prescribe any antibiotics.
Before choosing an antibiotic, your doctor will ask you about your symptoms and examine you. If relevant, he or she may ask if you have travelled abroad recently or if you have any problems with your immune system. If you’re vomiting or have diarrhoea, your doctor may ask you to provide a sample of faeces (also called a stool sample). These samples will be tested in a laboratory to see which bacteria are causing your infection, so you can be prescribed the correct antibiotic.
Your doctor may prescribe you a broad-spectrum antibiotic to take until he or she gets the results of your tests, or if he or she is unsure of the specific bacteria causing your infection. This type of antibiotic gets rid of infections caused by a wide range of bacteria. Broad-spectrum antibiotics aren't always the best solution, as using them can cause bacteria to become resistant. However, if you have a potentially serious infection, you will usually be given these so that your treatment isn’t delayed as this could lead to your illness getting worse.
If your doctor is confident which bacteria or type of bacteria is causing your infection, he or she may prescribe a narrow-spectrum antibiotic. This type of antibiotic only works against specific bacteria.
As well as considering which bacteria are most likely to be causing your infection, your doctor will consider your medical history and how severe your infection is to guide his or her choice of antibiotics. This may include whether you have any allergies to specific antibiotics, if you have any problems with your kidneys or liver and whether you're pregnant or breastfeeding. You may need to take more than one type of antibiotic at the same time.
If you need penicillin antibiotics but are allergic to them, your doctor will prescribe a different type for you.
Always ask your doctor or pharmacist for advice and read the patient information leaflet that comes with your medicine.
If I feel better after a few days, can I stop taking my antibiotics?
You should always finish your entire course of antibiotics even if you feel better after a few days. This is because the bacteria causing your infection may still be in your body and could flare up to cause another infection. If you stop antibiotics too early, you increase the risk of the bacteria becoming resistant to the antibiotic and this may make the infection more difficult to treat in the future. It also means that the infection may come back soon after.
To clear your infection completely you will need to continue taking your antibiotics for the full course of treatment, as directed by your doctor, nurse prescriber or dentist. You should do this even if you begin to feel better after a few days. If you stop taking your antibiotics too soon, your symptoms may return.
Finishing your course of antibiotics will also reduce the risk of antibiotic resistance developing. If the bacteria causing your infection aren't completely killed off, they can change in a way that makes them more difficult to cure with antibiotics. Some bacteria aren’t killed because they have a genetic mutation that allows them to survive and multiply. This can cause your infection to continue. If the bacteria causing the infection you're being treated for become resistant, your doctor will need to prescribe you a different antibiotic.
You’re likely to be prescribed antibiotics for a period of two to four or seven to 10 days, and you will usually need to take them at regular intervals throughout the day. It's very important that you take your antibiotics as prescribed and follow the directions on the medicine label. This will ensure that you have the correct amount of medicine in your body to kill off the bacteria causing your infection.
If you miss a dose of antibiotic, take it as soon as possible and then return to your usual dosing schedule. If it’s nearly time to take your next dose, don’t take the one you forgot and stick to your usual schedule. Don't double up on doses. If your antibiotics make you feel ill or you want to stop taking them for some reason, contact your doctor as soon as possible. Always ask your doctor for advice and read the patient information leaflet that comes with your medicine.
Why won't my GP give me antibiotics for my flu?
Antibiotics are only effective against infections caused by bacteria. Flu (influenza) is caused by a virus, so antibiotics won’t work and shouldn't be used for this type of infection. Antibiotics may sometimes be helpful if you develop a secondary bacterial infection when you have flu or are recovering from it.
You may be prescribed antibiotics to help your immune system fight bacterial infections but they don’t work against viral infections such as flu.
Your body may be weakened after fighting a flu infection, which makes it possible that you will pick up a secondary infection caused by bacteria, such as pneumonia. Your body may find it difficult to fight off this secondary infection so your doctor may have to prescribe antibiotics to help. Secondary infections can affect your chest, ears, nose or throat.
If you have flu, it can take up to two weeks to recover completely. The best treatment is usually bed rest, drinking enough fluids and taking over-the-counter painkillers. However, contact your GP if you're concerned about any of your symptoms, particularly if they don’t go away. If you’re particularly at risk of developing complications of flu, for example you’re over 65 or have a serious medical condition such as chronic obstructive pulmonary disease (COPD), having the flu vaccine each year can help prevent you from getting it. Speak to your GP for more information.
- British Medical Association. The BMA new guide to medicines and drugs. 5th ed. London: Dorling Kindersley; 2002
- Joint Formulary Committee. British National Formulary. 64th ed. London: British Medical Association and Royal Pharmaceutical Society of Great Britain; 2012
- Fact sheet: the safe use of antibiotics. Institute for Quality and Efficiency in Health Care. www.informedhealthonline.org, published January 2011
- Metronidazole tablets 500mg. electronic Medicines Compendium. www.medicines.org.uk, published April 2012
- Antibiotics. The Merck Manuals. www.merckmanuals.com, published September 2008
- Winter colds, flu and chest infections – differentiating between the wintry ailments. Totalhealth. www.totalhealth.co.uk, published November 2012
- Antibiotics don’t work on colds and flu. Health Service Executive. www.hse.ie, published November 2012
- Immunizations – seasonal influenza. Prodigy. www.prodigy.clarity.co.uk, published September 2011
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