What are antibiotics?
Antibiotics are used to treat or prevent some types of bacterial infection. They work by killing bacteria or preventing them from reproducing and spreading. But they don’t work for everything. When it comes to antibiotics, take your doctor’s advice.
Antibiotics don't work for viral infections such as colds and flu, and most coughs and sore throats.
Many mild bacterial infections also get better on their own without using antibiotics.
Taking antibiotics when you don’t need them encourages dangerous bacteria that live inside you to become resistant. That means that antibiotics may not work when you next need them most. This puts you and your family at serious risk.
When antibiotics are used
Antibiotics may be used to treat bacterial infections that:
- are unlikely to clear up without antibiotics
- could infect others unless treated
- could take too long to clear without treatment
- carry a risk of more serious complications
People at a high risk of infection may also be given antibiotics as a precaution, known as antibiotic prophylaxis.
How do I take antibiotics?
Take antibiotics as directed on the packet or the patient information leaflet that comes with the medication, or as instructed by your doctor or pharmacist.
Doses of antibiotics can be provided in several ways:
- Oral antibiotics – tablets, capsules or a liquid that you drink, which can be used to treat most types of mild to moderate infections in the body
- Topical antibiotics – creams, lotions, sprays or drops, which are often used to treat skin infections
- Injections of antibiotics – these can be given as an injection or infusion through a drip directly into the blood or muscle, and are usually reserved for more serious infections
It's essential to finish taking a prescribed course of antibiotics, even if you feel better, unless a healthcare professional tells you otherwise. If you stop taking an antibiotic part way through a course, the bacteria can become resistant to the antibiotic.
Missing a dose of antibiotics
If you forget to take a dose of your antibiotics, take that dose as soon as you remember and then continue to take your course of antibiotics as normal.
But if it's almost time for the next dose, skip the missed dose and continue your regular dosing schedule. Don't take a double dose to make up for a missed one.
There's an increased risk of side effects if you take two doses closer together than recommended.
Accidentally taking an extra dose
Accidentally taking one extra dose of your antibiotic is unlikely to cause you any serious harm.
But it will increase your chances of experiencing side effects, such as pain in your stomach, diarrhoea, and feeling or being sick.
If you accidentally take more than one extra dose of your antibiotic, are worried or experiencing severe side effects, speak to your doctor.
Side effects of antibiotics
As with any medication, antibiotics can cause side effects. Most antibiotics don't cause problems if they're used properly and serious side effects are rare.
The most common side effects include:
- being sick
- feeling sick
- bloating and indigestion
Some people may have an allergic reaction to antibiotics, especially penicillin and a type called cephalosporins. In very rare cases, this can lead to a serious allergic reaction (anaphylaxis) , which is a medical emergency.
Read more about the side effects of antibiotics.
Considerations and interactions
Some antibiotics aren't suitable for people with certain medical conditions, or women who are pregnant or breastfeeding. You should only ever take antibiotics prescribed for you – never "borrow" them from a friend or family member.
Some antibiotics can also react unpredictably with other medications, such as the oral contraceptive pill and alcohol. It's important to read the information leaflet that comes with your medication carefully and discuss any concerns with your pharmacist or doctor.
Read more about:
Types of antibiotics
There are hundreds of different types of antibiotics, but most of them can be broadly classified into six groups. These are outlined below.
- Penicillins (such as penicillin and amoxicillin) – widely used to treat a variety of infections, including skin infections, chest infections and urinary tract infections
- Cephalosporins (such as cephalexin) – used to treat a wide range of infections, but some are also effective for treating more serious infections, such as septicaemia and meningitis
- Aminoglycosides (such as gentamicin and tobramycin) – tend to only be used in hospital to treat very serious illnesses such as septicaemia, as they can cause serious side effects, including hearing loss and kidney damage; they're usually given by injection, but may be given as drops for some ear or eye infections
- Tetracyclines (such as tetracycline and doxycycline)– can be used to treat a wide range of infections, but are commonly used to treat moderate to severe acne and rosacea
- Macrolides (such as erythromycin and clarithromycin) – can be particularly useful for treating lung and chest infections, or an alternative for people with a penicillin allergy, or to treat penicillin-resistant strains of bacteria
- Fluoroquinolones (such as ciprofloxacin and levofloxacin) – broad-spectrum antibiotics that can be used to treat a wide range of infections
Health organisations across the world are trying to reduce the use of antibiotics, especially for conditions that aren't serious.
The overuse of antibiotics in recent years means they're becoming less effective and has led to the emergence of "superbugs". These are strains of bacteria that have developed resistance to many different types of antibiotics, including:
- methicillin-resistant Staphylococcus aureus (MRSA)
- Clostridium difficile (C. diff)
- the bacteria that cause multi-drug-resistant tuberculosis (MDR-TB)
- carbapenemase-producing Enterobacteriaceae (CPE)
These types of infections can be serious and challenging to treat, and are becoming an increasing cause of disability and death across the world.
The biggest worry is that new strains of bacteria may emerge that can't be effectively treated by any existing antibiotics.
What are antibiotics used for?
Antibiotics are used to treat or prevent some types of bacterial infections. They aren't effective against viral infections, such as the common cold or flu.
Antibiotics should only be prescribed to treat conditions:
- that aren't especially serious but are unlikely to clear up without the use of antibiotics – such as moderately severe acne
- that aren't especially serious but could spread to other people if not promptly treated – such as the skin infection impetigo or the sexually transmitted infection
- where evidence suggests that antibiotics could significantly speed up recovery – such as a kidney infection
- that carry a risk of more serious complications – such as cellulitis or pneumonia
Antibiotics are no longer routinely used to treat infections because:
- many infections are caused by viruses, so antibiotics aren't effective
- antibiotics are often unlikely to speed up the healing process and can cause side effects
- the more antibiotics are used to treat trivial conditions, the more likely they are to become ineffective for treating more serious conditions
People at risk of bacterial infections
Antibiotics may also be recommended for people who are more vulnerable to the harmful effects of infection. This may include:
- people aged over 75 years
- babies less than 72 hours old with a confirmed bacterial infection, or a higher than average risk of developing one
- people with heart failure
- people who have to take insulin to control their diabetes
- people with a weakened immune system – either because of an underlying health condition such as HIV infection or as a side effect of certain treatments, such as chemotherapy
Antibiotics to prevent infection
Antibiotics are sometimes given as a precaution to prevent, rather than treat, an infection. This is known as antibiotic prophylaxis.
Antibiotic prophylaxis is normally recommended if you're having surgery on a certain part of the body which carries a high risk of infection or where infection could lead to devastating effects.
For example, it may be used if you're going to have:
- some types of eye surgery – such as cataract surgery or glaucoma surgery
- joint replacement surgery
- breast implant surgery
- pacemaker surgery
- surgery to remove the gall bladder
- surgery to remove the appendix
Your surgical team will be able to tell you if you require antibiotic prophylaxis.
Bites or wounds
Antibiotic prophylaxis may be recommended for a wound that has a high chance of becoming infected – this could be an animal or human bite , for example, or a wound that has come into contact with soil or faeces.
There are several medical conditions that make people particularly vulnerable to infection, making antibiotic prophylaxis necessary.
For example, the spleen plays an important role in filtering out harmful bacteria from the blood. People who have had their spleen removed, people having chemotherapy for cancer, or those with the blood disorder sickle cell anaemia , where their spleen doesn't work properly, should take antibiotics to prevent infection.
In some cases, antibiotic prophylaxis is prescribed for people who experience a recurring infection that's causing distress or an increased risk of complications, such as:
What do I need to know before taking antibiotics?
There are some important things to consider before taking antibiotics.
This page contains information on the six main classes of antibiotics:
Don't take one of the penicillin-based antibiotics if you've had an allergic reaction to them in the past. People who are allergic to one type of penicillin will be allergic to all of them.
Penicillins may need to be used at lower doses and with extra caution if you have:
Pregnancy and breastfeeding
Most penicillins can be used during pregnancy and breastfeeding in the usual doses.
Tell your healthcare professional if you're pregnant or breastfeeding, so they can prescribe the most suitable antibiotic for you.
If you previously had an allergic reaction to penicillin, there's a chance that you may also be allergic to cephalosporins.
Cephalosporins may not be suitable if you have kidney disease, but if you need one you will probably be given a lower than usual dose.
If you're pregnant or breastfeeding, or have acute porphyria, check with your doctor, midwife or pharmacist before taking cephalosporins.
Aminoglycosides are normally only used in hospital to treat life-threatening conditions such as septicaemia, as they can cause kidney damage in people with pre-existing kidney disease.
They're only used during pregnancy if your doctor believes they're essential.
The use of tetracyclines isn't usually recommended unless absolutely necessary in the following groups:
- people with kidney disease – except doxycycline, which can be used
- people with liver disease
- people with the autoimmune condition lupus – which can cause skin problems, joint pain and swelling, and fatigue (feeling tired all the time)
- children under the age of 12
- pregnant or breastfeeding women
You shouldn't take macrolides if you have porphyria – a rare inherited blood disorder.
If you're pregnant or breastfeeding, the only type of macrolide you can take is erythromycin (Erymax, Erythrocin, Erythroped or Erythroped A) unless a different antibiotic is recommended by your doctor.
Erythromycin can be used at the usual doses throughout your pregnancy and while you're breastfeeding.
Other macrolides shouldn't be used during pregnancy, unless advised by a specialist.
Fluoroquinolones aren't normally suitable for women who are pregnant or breastfeeding.
Can antibiotics cause side effects?
The most common side effects of antibiotics affect the digestive system. These occur in around 1 in 10 people.
Side effects of antibiotics that affect the digestive system include:
- nausea (feeling like you may vomit)
- bloating and indigestion
- abdominal pain
- loss of appetite
These side effects are usually mild and should pass once you finish your course of treatment.
If you experience any additional side effects other than those listed above, you should contact your doctor or the doctor in charge of your care for advice.
Antibiotic allergic reactions
Around 1 in 15 people have an allergic reaction to antibiotics, especially penicillin and cephalosporins. In most cases, the allergic reaction is mild to moderate and can take the form of:
- a raised, itchy skin rash (urticaria, or hives)
- tightness of the throat, which can cause breathing difficulties
These mild to moderate allergic reactions can usually be successfully treated by taking antihistamines.
But if you're concerned, or your symptoms don't respond to treatment, you should call your doctor for advice. If you can't contact your doctor, call the emergency services.
In rare cases, an antibiotic can cause a severe and potentially life-threatening allergic reaction known as anaphylaxis.
Initial symptoms of anaphylaxis are often the same as above and can lead to:
- a rapid heartbeat
- increasing difficulty breathing caused by swelling and tightening of the neck
- a sudden intense feeling of apprehension and fear
- a sharp and sudden drop in your blood pressure, which can make you feel light-headed and confused
- falling unconsciousness
Anaphylaxis is a medical emergency and can be life-threatening if prompt treatment isn't given. Call emergency services immediately and ask for an ambulance if you think you or someone around you is experiencing anaphylaxis.
Tetracyclines and sensitivity to light
Tetracyclines can make your skin sensitive to sunlight and artificial sources of light, such as sun lamps and sunbeds.
You should avoid prolonged exposure to bright light while taking these drugs.
What should I avoid when taking antibiotics?
Antibiotics can sometimes interact with other medicines or substances. This means it can have an effect that is different to what you expected.
Some of the more common interactions are listed below, but this isn't a complete list.
If you want to check that your medicines are safe to take with your antibiotics, ask your GP or local pharmacist.
Some antibiotics need to be taken with food, while others need to be taken on an empty stomach. You should always read the patient information leaflet that comes with your medicine.
You should completely avoid alcohol while taking the antibiotics metronidazole or tinidazole, and for 48 hours afterwards, as this combination can cause very unpleasant side effects, such as:
- feeling and being sick
- stomach pain
- hot flushes
It's recommended that you don't drink alcohol while taking antibiotics. However, as long as you drink in moderation, alcohol is unlikely to interact significantly with your medication.
Combined oral contraceptives
Some antibiotics, such as rifampicin and rifabutin, can reduce the effectiveness of the combined oral contraceptive pill .
If you're prescribed rifampicin or rifabutin, you may need to use additional contraception, such as condoms, while taking antibiotics. Speak to your GP, nurse or pharmacist for advice.
Some of the medications you may need to avoid, or seek advice on, while taking a specific class of antibiotic are outlined below.
It's usually recommended that you avoid taking penicillin at the same time as methotrexate, which is used to treat psoriasis , rheumatoid arthritis and some forms of cancer. This is because combining the two medications can cause a range of unpleasant and sometimes serious side effects.
However, some forms of penicillin, such as amoxicillin, can be used in combination with methotrexate.
You may experience a skin rash if you take penicillin and allopurinol, which is used to treat gout.
Cephalosporins may increase the chance of bleeding if you're taking blood-thinning medications (anticoagulants) such as heparin and warfarin .
If you need treatment with cephalosporins, you may need to have your dose of anticoagulants changed or additional blood monitoring.
The risk of damage to your kidneys and hearing is increased if you're taking one or more of the following medications:
- antifungals – used to treat fungal infections
- cyclosporin – used to treat autoimmune conditions such as Crohn's disease and given to people who have had an organ transplant
- diuretics – used to remove water from the body
- muscle relaxants
The risk of kidney and hearing damage has to be balanced against the benefits of using aminoglycosides to treat life-threatening conditions such as septicaemia.
In hospital, blood levels are carefully monitored to ensure the antibiotic is only present in the blood in safe amounts. If aminoglycosides are used properly in topical preparations, such as ear drops, these side effects don't occur.
You should check with your GP or pharmacist before taking a tetracycline if you're currently taking any of the following:
- vitamin A supplements
- retinoids – such as acitretin, isotretinoin and tretinoin, which are used to treat severe acne
- blood-thinning medication
- kaolin-pectin and bismuth subsalicylate – used to treat diarrhoea
- medicines to treat diabetes – such as insulin
- atovaquone – used to treat pneumonia
- antacids – used to treat indigestion and heartburn
- sucralfate – used to treat ulcers
- lithium – used to treat bipolar disorder and severe depression
- digoxin – used to treat heart rhythm disorders
- strontium ranelate – used to treat osteoporosis
- colestipol or colestyramine – used to treat high cholesterol
- ergotamine and methysergide – used to treat migraines
It's highly recommended that you don't combine a macrolide with any of the following medications unless directly instructed to by your GP, as the combination could cause heart problems:
- terfenadine, astemizole and mizolastine – these are all antihistamines used to treat allergic conditions such as hay fever
- amisulpride – used to treat episodes of psychosis
- tolterodine – used to treat urinary incontinence
- statins – used to treat high cholesterol
You should check with your GP or pharmacist before taking a fluoroquinolone if you're currently taking any of the following:
- theophylline – used to treat asthma; also found in some cough and cold medicines
- non-steroidal anti-inflammatory drug (NSAID) painkillers – such as ibuprofen
- probenecid – used to treat gout
- clozapine – used to treat schizophrenia
- ropinirole – used to treat Parkinson's disease
- tizanadine – used to treat muscle spasms
- glibenclamide – used to treat diabetes
- cisapride – used to treat indigestion, heartburn, vomiting or nausea
- tricyclic antidepressants – such as amitriptyline
- steroid medications (corticosteroids)
Some fluoroquinolones can intensify the effects of caffeine (a stimulant found in coffee, tea and cola), which could make you feel irritable, restless and cause problems falling asleep (insomnia).
You may need to avoid taking medication that contains high levels of minerals or iron, as this can block the beneficial effects of fluoroquinolones. This includes:
- zinc supplements
- some types of multivitamin supplements