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Antibiotics

Expert reviewer, Madeeha Waheed, Oncology Pharmacist at Bupa, Clinical and Operational Improvement
Next review due April 2025

Antibiotics are medicines used to treat or prevent some types of bacterial infection. It’s important that antibiotics are taken in the right way, and only when they’re really needed. This can help to prevent them becoming less effective (antibiotic resistance).

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Uses of antibiotics

There are strict rules about when your doctor can offer you antibiotics. These include if you have an infection thought to be caused by bacteria and one or more of the following apply.

  • You are very unwell with symptoms that suggest you have a severe bacterial infection.
  • Your infection gets suddenly worse or doesn’t improve after a certain period of time. Sometimes, your doctor may give you a ‘back-up’ or ‘delayed’ prescription. This means you can get the antibiotics at a later date, if your infection hasn’t cleared up.
  • You’re at high risk of developing complications. This may include if you have a weakened immune system (you’re immunocompromised) or if you have certain other health conditions.
  • You have an infection that can be easily spread to other people. These include certain skin infections and sexually transmitted infections (STIs). Treatment with antibiotics can help to reduce the risk of these spreading.
  • You have a serious bacterial infection such as pneumonia, meningitis or sepsis. Treatment with antibiotics in these cases can be lifesaving.

Mild bacterial infections such as ear infections, sinusitis and sore throats, often clear up by themselves. So, your doctor won’t usually prescribe antibiotics for these straight away. Your doctor also won’t treat viral illnesses such as colds, flu or chickenpox with antibiotics because they don’t work for viral infections.

Taking antibiotics when they’re not really needed can mean they become less effective and may not work when we do need them. For more information, see our section on antibiotic resistance.

Antibiotics can be used to prevent bacterial infections. This is why your doctor may offer you antibiotics before and after certain types of surgery. You may also be offered antibiotics if you have a weakened immune system (for example, during chemotherapy or if you have HIV infection).

How antibiotics work?

Antibiotics work in two ways. They either kill bacteria or stop the bacteria from growing and multiplying. They often do both. Antibiotics interfere with different processes in the cells of the bacteria that they target. This stops the bacteria from growing or functioning properly. Some antibiotics work better the larger a dose you have, others are more effective the longer you take them.

Different antibiotics target different types of bacteria. That’s why your doctor will usually try to find out the type of bacteria causing your infection before prescribing an antibiotic. This makes sure that you have the most effective antibiotic for your infection.

Types of antibiotics

There are different groups of antibiotics. These work in different ways and target different types of bacteria. The main groups include:

  • penicillins (for example, amoxicillin, phenoxymethylpenicillin (Penicillin V), flucloxacillin and co-amoxiclav).
  • cephalosporins (for example, cefalexin and cefaclor).
  • tetracyclines (for example, doxycycline and minocycline)
  • aminoglycosides (for example, gentamicin and tobramycin)
  • macrolides (for example, erythromycin and clarithromycin)
  • fluoroquinolones (for example, ciprofloxacin and levofloxacin).

Other common antibiotics include nitrofurantoin and trimethoprim. These are often used for urine infections.

Antibiotics can also be classed as either broad-spectrum or narrow-spectrum. Broad-spectrum antibiotics can treat infections caused by a wide range of different bacteria. Narrow-spectrum antibiotics only work against specific bacteria.

When prescribing an antibiotic, your doctor will look at several factors to decide which is the most appropriate one for you. These include the type of infection you have, the part of your body affected and results of any laboratory tests. They’ll also consider your age and medical history, including if you’ve previously taken antibiotics for an infection. Tell your doctor about any health conditions you have or medicines you’re taking. You should also let them know if you’re pregnant or breastfeeding.

The leaflet that comes with your medicine will have more information. It will tell you what type of antibiotic you’re taking and what conditions it’s usually used for.

Taking antibiotics

What forms do antibiotics come in?

Antibiotics come in different forms including:

  • tablets or capsules (the most common forms)
  • liquids (for children or people who have difficulty swallowing)
  • ear and eye drops (for example, for conjunctivitis)
  • creams (for skin conditions such as eczema and impetigo)
  • injections/infusions into a vein (for serious infections – you’d usually have these in hospital)

Usually, you only get antibiotics on prescription from a doctor. However, you can buy chloramphenicol eye drops (for conjunctivitis) from a pharmacy without a prescription.

Using antibiotics correctly

It’s important not to misuse antibiotics. This is because misuse can encourage the development of antibiotic resistance in bacteria. This can lead to longer and more severe illness. Make sure you follow these points when taking antibiotics.

  • Take your antibiotics for exactly as long as directed in your prescription. Finish the whole course, even if you start to feel better. This will make sure all the harmful bacteria are gone. It will also help stop the infection returning.
  • Don’t take prescription-only antibiotics if they haven’t been prescribed for you by a qualified health professional. This means do not buy antibiotics online without a prescription. And don’t take someone else’s antibiotics or share yours with another person.
  • Don’t ask for or take antibiotics ‘just in case’ (when you’re travelling abroad, for instance) unless your doctor has advised you to. Your doctor may suggest this if you’re at particular risk of infection.
  • Don’t keep leftover antibiotics to use later or flush them down the toilet or sink – hand any unused antibiotics to your pharmacist for disposal. You should only have leftover antibiotics if your doctor has advised you stop taking them.

The leaflet that comes with your medicine will give you lots of important information about when and how to take your antibiotics. Your doctor or pharmacist may also give you advice. It’s important to follow any instructions you’re given.

Taking antibiotics with other medicines and alcohol

Some antibiotics can cause difficulties if you take them with other medicines, foods or drinks. You might get a bad reaction from having the two things together. Sometimes, combinations of medicines affect how well one or the other works. You’ll find details of any substances you should avoid in the patient information leaflet that comes with your medicine. Ask your pharmacist if you have any questions. Here are two important examples.

Antibiotics and the contraceptive pill

Certain antibiotics (for example, rifampicin) can stop the contraceptive pill from working as well as it should. Some antibiotics can cause diarrhoea and vomiting, and this may stop your contraceptive pill from being absorbed properly. Check with your doctor if you’re taking the contraceptive pill. You may need to use another method of contraception while you’re on antibiotics.

Antibiotics and alcohol

If you’re feeling unwell, it’s usually best to avoid alcohol for a while. But it’s unlikely that you’ll have any problems with most common antibiotics if you drink moderate amounts of alcohol. But you should avoid alcohol completely if you’re taking an antibiotic called metronidazole. The combination of metronidazole and alcohol can cause severe side-effects including hot flushes, vomiting and a fast heartbeat. Don’t drink any alcohol for at least two days after taking metronidazole.

Antibiotic side-effects

Side-effects are the unwanted effects of taking a medicine. It’s not possible to list all the side-effects of all the different antibiotics here. The patient information leaflet that comes with your medicine will list these, so you should read this carefully.

Many of the side-effects of antibiotics happen because of their effect on the natural bacteria that live in our bodies. As well as killing harmful bacteria, they also destroy these ‘friendly’ protective bacteria. This can lead to an overgrowth of harmful bacteria or fungi, causing problems such as thrush and diarrhoea. Other side-effects of antibiotics can include rashes, stomach pains and reactions to sunlight.

If you have side-effects, it’s important to talk to your doctor before you stop taking the medicine. Depending on your infection, they may be able to offer you an alternative antibiotic.

Allergies to antibiotics

A more serious possible side-effect of taking antibiotics is an allergic reaction. This is most likely with penicillins and cephalosporins. Allergic reactions range from a mild skin rash to a severe life-threatening reaction. If you’ve had an allergic reaction to an antibiotic in the past, be sure to tell your doctor.

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

  • an itchy rash
  • flushing of your skin
  • swelling in any part of your body but especially your face, throat and tongue
  • feeling or being sick

If you notice any of these reactions after taking your antibiotic, stop taking it and contact your GP as soon as possible.

Occasionally, the allergic reaction may be more severe, leading to symptoms such as:

  • difficulty breathing
  • wheezing
  • feeling light-headed or faint
  • a fast heartbeat

Seek immediate medical attention if you develop any of these symptoms after taking antibiotics.

Allergy to penicillins is quite rare. About one in 10 people think they have an allergy to penicillin, but for most people this isn’t the case. They may have just had side-effects like sickness or a headache, or a mild skin rash as a child.

Antibiotic resistance

Bacteria can develop resistance to antibiotics. This means that the antibiotic no longer works as well and may not be able to treat the infection. The more you take an antibiotic, the more bacteria become resistant to it. These resistant bacteria can then cause a further infection or even spread to other people.

Some bacteria can become resistant to several antibiotics at the same time. It can be much harder to treat infections with these antibiotic-resistant bacteria. One example of antibiotic-resistant bacteria is MRSA (methicillin-resistant Staphylococcus aureus). MRSA can cause serious infections and is resistant to most of the usual antibiotics your doctor would prescribe.

Using antibiotics when they’re not really needed encourages resistant bacteria to develop. This can affect you, as well as other people that the bacteria may spread to. Working with your doctor to make sure you use antibiotics correctly can help to prevent antibiotic resistance.

Medicines checklist

Our handy medicines checklist helps you see what to check for before taking a medicine.

Bupa's medicines checklist PDF opens in a new window (0.8MB)

Bupa medicines checklist

Frequently asked questions


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Related information

  • Discover other helpful health information websites

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  • Reviewed by Pippa Coulter, Freelance Health Editor, April 2022
    Expert reviewer, Madeeha Waheed, Oncology Pharmacist at Bupa, Clinical and Operational Improvement
    Next review due April 2025

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