Expert reviewer, Dr Adrian Raby, General Practitioner
Next review due August 2020

Antibiotics are medicines used to treat infections caused by bacteria. They are used for many things, from an ear infection to more serious conditions like pneumonia or meningitis.

They’re effective in easing symptoms and speeding up recovery, and can save lives. But you need to use antibiotics correctly. This will help to prevent the increasing problem of antibiotic resistance – when antibiotics stop working against bacteria.

A pair of slippers

Why has my doctor advised me to take antibiotics?

Your doctor will prescribe antibiotics to treat an infection that’s caused by bacteria. Common bacterial infections include those affecting your ear, chest, urinary system and skin. Some sexually transmitted infections (STIs), for example gonorrhoea and chlamydia, are also caused by bacteria.

Some bacterial infections clear up by themselves, so your doctor may want to wait before prescribing antibiotics you may not need. They’ll advise you on other treatment and self-management that could help.

Many infectious illnesses are caused by viruses, not bacteria. These include flu, the common cold and chicken pox. Antibiotics don’t work against viruses, so your doctor won’t normally prescribe them to treat these illnesses.

Your medical history and an examination can help your doctor decide whether your infection is caused by a virus or bacteria. They may want to take samples of your blood, urine, faeces or mucus, or a swab (from your throat or skin, for example). Laboratory tests on these will help show what’s causing your infection. If it’s bacteria, the results will help your doctor pick the most effective antibiotic to treat you.

Sometimes, a viral infection leads to what’s called a secondary bacterial infection, such as pneumonia. In circumstances like this, your doctor may prescribe antibiotics to treat the secondary bacterial infection. For more information on treating complications of flu and colds, see our FAQ: Why won't my doctor give me antibiotics for a cold or flu?

You may be given antibiotics to prevent an infection after certain types of surgery or if you’re at particular risk of certain infections.

Some bacterial infections don’t respond to antibiotics alone. For instance, if you have an abscess, you may also need to have it drained.

What are the main types of antibiotic?

Your doctor will try to find the antibiotic that suits you best. There are lots of different types that work in different ways. These include penicillins, tetracyclines, macrolides and cephalosporins.

Broad-spectrum antibiotics

A broad-spectrum antibiotic can treat infections caused by a wide range of different bacteria. Penicillins are generally this type.

Your doctor may prescribe you a broad-spectrum antibiotic until the results of any tests come back. They may also prescribe this type of antibiotic if your infection is caused by more than one type of bacteria. Broad-spectrum antibiotics aren’t always the best solution, because using them may cause bacteria to become resistant.

However, if you have a potentially serious infection, you will usually be given these. This is so your treatment isn’t delayed, which could lead to your illness getting worse.

Narrow-spectrum antibiotics

If your doctor thinks a specific bacteria or type of bacteria is causing your infection, they may prescribe a narrow-spectrum antibiotic. This type of antibiotic only works against specific bacteria.

If necessary, your doctor will order tests to see exactly what bacteria is causing your infection and choose a narrow-spectrum antibiotic to target it. But if they want to start your treatment immediately, without waiting for test results, they’ll usually prescribe a broad-spectrum antibiotic.

Always read the patient information leaflet that comes with your medicine. If you have any questions, ask your pharmacist or doctor for advice.

How do antibiotics work?

Some antibiotics work by killing bacteria. They usually do this by destroying the cell walls of bacteria. Penicillins work 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 start taking these medicines.

How to take antibiotics

Antibiotics come in different forms including:

  • tablets or capsules (most commonly)
  • liquids (if you have difficulty swallowing or when giving to children)
  • ear and eye drops (eg for conjunctivitis)
  • creams (for skin conditions like eczema and broken skin which gets infected)
  • injections (for serious infections – these are usually given in hospital)

Usually you can only get antibiotics on prescription. However, some you can buy at the pharmacy over the counter (such as eye drops for conjunctivitis).

How long do I take them for?

You’ll need to take antibiotics for a set period of time, which could be days or weeks. Your prescription should say how long you should take your medicine for, or when your treatment will be reviewed.

The length of your course of antibiotics depends on the type of infection you have. For instance, a three-day course may clear up cystitis and other urinary tract infections (UTIs) that need antibiotics. But if you have a serious infection such as osteomyelitis, you could have to take antibiotics for weeks or months to clear it up.

It’s important to finish the whole course, even if you start to feel better. This will make sure all the bacteria are gone and stop them becoming resistant to the medicine.

Your doctor may want to check your infection has completely cleared up with tests and an examination. For more information on why you shouldn’t stop before the end of your course of antibiotics, see our FAQ: Can I stop taking my antibiotics once I feel better?

Make sure you follow the advice your doctor gives you and read the patient information leaflet that comes with your medicine.

When should I take them?

You may have to follow specific instructions about exactly when to take your antibiotics. Taking your antibiotics at regular intervals – your doctor will tell you how often – means there’s a steady level of the medicine in your body.

Some antibiotics should be taken before, with or after a meal as well as taking them at specific times. For example, you might need to take your medicine four times a day on an empty stomach, so you’ll need to factor this in when timing your meals. Ask your doctor if you have any questions. It’s usually best to take antibiotics in tablet form with water.

It’s important not to misuse antibiotics by:

  • taking someone else’s
  • sharing yours with another person
  • taking them ‘just in case’ (when you’re travelling abroad, for instance) unless your doctor has advised you to because you’re at particular risk of infection
  • not finishing your prescribed course and keeping any left over to use another time or flushing them down the toilet or sink

Follow the instructions on the medicine label or patient information leaflet. This will tell you what to do if you miss a dose or think you’ve taken too much. If you have any questions, ask your doctor or pharmacist for advice.

Special care

Liver and kidney problems

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

Pregnant or breastfeeding

It’s also important to tell your health professional if you’re pregnant or might be, if you’re planning to get pregnant or are breastfeeding. Some antibiotics are safe to use in pregnancy, but others aren’t. Most will get into your breast milk and could affect your baby.


You’ll need strong antibiotics urgently if your doctor suspects your infection has developed into sepsis. This is a very serious condition when your body responds to infection and the response results in serious damage to tissues and vital organs. To start with, you’ll have an injection of broad-spectrum antibiotics within an hour, as well as other supportive measures such as oxygen. Results from blood and other tests can help show exactly where and what the infection is. This helps your doctors decide whether your antibiotic should then be switched to one with a more narrow spectrum.

Side-effects of antibiotics

Side-effects are the unwanted effects of taking a medicine. Some of the most common side-effects of antibiotics include diarrhoea and feeling sick or vomiting but you may not necessarily have any of these.

Antibiotic eye drops can sting and blur your vision when you put them in. Ear drops may make the inside of your ear inflamed and affect your hearing.

It’s possible that you may get a fungal infection such as thrush after you take broad-spectrum antibiotics. This is because as well as killing the harmful bacteria that are causing your infection, antibiotics may also destroy your body’s protective bacteria. These normally stop micro-organisms such as fungi growing out of control. If you develop a fungal infection, you may need to take an antifungal medicine to treat it.

You may also develop bowel problems if antibiotics affect a type of bacteria called Clostridium difficile found in your intestine. This can increase the levels of toxins normal bacteria produce and lead to more serious infection. Older people and those with a weakened immune system are most at risk.

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

If you have side-effects, it’s important to talk to your doctor or the healthcare professional who prescribed your medicine before you stop taking it. Depending on your infection, they may be able to offer you an alternative antibiotic, although this won’t always be possible.

Allergies to antibiotics

A more serious side-effect of taking antibiotics is having an allergic reaction. However, true allergy to antibiotics is quite rare. About one in 10 people think they have an allergy to penicillin, but fewer that one in 10 of those people have a true allergy. The reason people think they have an allergy when they perhaps don’t is because they got a rash when taking penicillin as a child. But if the rash was minor or only appeared after three days of taking penicillin, then it’s likely not to be an allergy. If this is what happened to you and you get a serious infection, your doctor may suggest you take penicillin because it’s the best medicine to treat your illness. They will consider this carefully though because a severe reaction to penicillin is potentially fatal.

If you’ve had an allergic reaction in the past, be sure to tell your doctor. If you’re at risk of having a severe reaction, you won’t be able to have that particular type of antibiotic so your doctor will prescribe a different one for you.

If you’re allergic to an antibiotic, typical symptoms include:

  • an itchy rash
  • sore eyes
  • a blocked or runny nose
  • swelling in any part of your body but especially your face, throat and tongue
  • difficulty breathing
  • a faster heartbeat
  • feeling or being sick
  • feeling faint

These reactions usually happen very quickly after taking antibiotics.

If you have a severe allergic reaction, this is called anaphylaxis. The symptoms of anaphylaxis include a swollen face, throat and tongue and difficulty breathing. Your blood pressure may fall and you may lose consciousness. It’s important to be aware that anaphylaxis can be serious, or even fatal. Seek immediate medical attention if you think you’re having these symptoms after taking antibiotics or any other medicines.

If you have a reaction, make a note of what happened, and what medicine you took and how much.

It’s possible you might get a referral to a specialist allergy clinic to find out exactly what you’re allergic to so you can avoid it in future.

Interactions with other medicines, food and drink

Certain antibiotics (eg rifampicin) can stop the contraceptive pill from working. Some side-effects from taking antibiotics (eg diarrhoea and vomiting) may stop your contraceptive pill from being absorbed into your body. If you’re taking antibiotics that affect the contraceptive pill or you’ve vomited or had diarrhoea, it’s important to use extra barrier contraception such as a condom. Doctors recommend doing this while you’re taking the antibiotics and for seven days after you’ve finished the course.

Antibiotics can interact with a number of other types of medicine such as blood thinners and antacids. Antibiotics may make the action of other medicines stronger or weaker. Before you take anything else at the same time as antibiotics, including herbal remedies, check with your pharmacist or health professional who prescribed the antibiotics.

You should also check if it’s OK to have certain foods or drinks if you’re taking antibiotics. For example, dairy products (including milk) and fruit juice can affect how your body absorbs some antibiotics. So does alcohol, which can also cause a reaction with some antibiotics, particularly one called metronidazole. Metronidazole is used to treat a number of infections including dental infections. Dietary supplements containing minerals like calcium may make your antibiotic less effective.

Antibiotic resistance

Some bacteria are resistant to certain antibiotics. This means that the antibiotic is no longer any good at getting rid of infection caused by these bacteria. For example, if you get infected with meticillin-resistant Staphylococcus aureus (MRSA), it won’t be possible to treat you with standard antibiotics.

Health professionals are trying to slow down the development of resistance, which is caused by the over-use and misuse of antibiotics. Your doctor or pharmacist may advise how you can manage an infection without antibiotics. They can also suggest ways to avoid infection, like handwashing and food hygiene.

Resistance can start if the bacteria that have caused an infection aren't completely killed off by an antibiotic. There’s more risk the infection will come back and the bacteria that survive may adapt so they’re resistant to that particular antibiotic. Bacteria may also pass on resistance when they multiply. This is why it’s really important to complete a course of antibiotics, even if you feel better. It’s the best way to prevent any bacteria from surviving.

Using antibiotics when you don’t really need them, especially broad-spectrum ones, means there’s more risk a wide variety of bacteria will become resistant. Some multi-resistant bacteria are resistant to several antibiotics at the same time.

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

Frequently asked questions

  • Always finish your entire course of antibiotics, even if you feel better after a few days, to clear your infection completely. 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 aren’t completely killed off, they can change in a way that makes them more difficult to kill 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 become resistant, your doctor will need to prescribe you a different antibiotic.

  • Antibiotics only work against infections caused by bacteria. Flu and colds are caused by viruses, so antibiotics won’t work and shouldn’t be used for these infections.

    Your body may be weakened after fighting a cold or flu infection. This can make you vulnerable to picking up a secondary infection caused by bacteria, such as pneumonia. Your body may find it difficult to fight this off so your doctor may have to prescribe antibiotics to help.

    If you have flu, it can take a week or two to recover completely. The best treatment is to rest if you need to (but try and stay sitting up rather than lying down), drink enough fluids and take over-the-counter painkillers. However, contact your GP if you’re concerned about your symptoms, particularly if they get worse. If you’re at risk of developing complications of flu, it’s a good idea to have the flu vaccine each year to help prevent getting it. You’re more at risk if you’re over 65 or are pregnant or if you:

    • have a lung disease such as asthma or chronic obstructive pulmonary disease (COPD)
    • have heart disease
    • have diabetes
    • have a weakened immune system
    • have liver or kidney disease
    • have had a stroke
    • have had your spleen removed

    Contact your GP for more information.

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  • Reviewed by Natalie Heaton, Specialist Health Editor, Bupa Health Content Team, August 2017
    Expert reviewer, Dr Adrian Raby, General Practitioner
    Next review due August 2020

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