Antibiotics

Antibiotics are medicines used to treat or prevent infections with bacteria. They don’t work for infections with viruses. It’s important that you take antibiotics in the right way, and only when you really need them. This helps to prevent them becoming less effective (antibiotic resistance).

Uses of antibiotics

There are strict rules about when your doctor can offer you antibiotics. These include having an infection that your doctor thinks is caused by bacteria and:

• you are very unwell with symptoms that suggest you have a severe bacterial infection
• your infection gets suddenly worse or doesn’t improve
• you have a high risk of complications because your immune system is weakened (immunocompromised)
• you have another health condition that puts you at greater risk from your infection
• your infection could spread to others, for example a skin infection or sexually transmitted infection (STI)
• you have a serious bacterial infection that could be life-threatening, such as pneumonia, meningitis, or sepsis

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.

Mild bacterial infections often clear up by themselves. These include ear infections, sinusitis, and sore throats. So, your doctor won’t usually prescribe antibiotics for these straightaway. Your doctor also won’t treat viral illnesses with antibiotics because they won’t work. Common viral infections include colds, flu, and chickenpox.

Taking antibiotics when you don’t need them can mean they stop working so well. Then they may not work when we really do need them. For more information, see our section on antibiotic resistance.

Antibiotics can also prevent bacterial infections. This is why your doctor may offer you antibiotics before and after certain types of surgery. You may also have them if you have a weakened immune system. For example, you may have them if you are having chemotherapy or have HIV infection.

How antibiotics work?

Antibiotics work in two main ways. They either kill bacteria or stop them from growing and multiplying. They often do both. Antibiotics can interfere with processes inside the bacterial cells they target. This stops the bacteria from multiplying or functioning properly. Some antibiotics need a minimum dose to work properly – in other words, you have to take a large enough dose. Others work best if you take them for a minimum amount of time – in other words, you need to take them for long enough.

Different antibiotics target different types of bacteria. Your doctor will often try to find out which type of bacteria is causing your infection before prescribing an antibiotic, particularly for a serious infection. This makes sure that you have the antibiotic that works best for the type of bacteria causing the 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, Penicillin V, and flucloxacillin)
• 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.

Your doctor will look at several factors to decide which antibiotic is best for you. These include:

• the type of infection you have
• the part of your body affected
• the results of any laboratory tests
• your age and medical history
• whether you’ve already had antibiotics for your 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 with other medicines and alcohol

Some antibiotics cause difficulties when taken with other medicines, foods, or drinks. You might get a bad reaction from having the two things together. Or sometimes combinations of medicines affect how well one or the other works. The patient information leaflet will tell you if there’s anything you should avoid. Ask your pharmacist if you have any questions. Here are two important examples.

Antibiotics and the contraceptive pill

Some antibiotics can stop the contraceptive pill from working as well as it should. One example is rifampicin.

Some antibiotics can cause diarrhoea and vomiting. This can stop your contraceptive pill from being absorbed properly. Check with your doctor. If you have diarrhoea or vomiting, 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.

You should avoid alcohol completely if you’re taking an antibiotic called metronidazole. Taken with alcohol, it can cause severe vomiting as well as hot flushes 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, you may just have a mild rash, which may be itchy. If this comes on at some point after you started taking your antibiotic, stop taking it and contact your GP as soon as possible. Your doctor will probably prescribe another antibiotic.

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

• a severe rash that comes on straightaway
• difficulty breathing
• wheezing
• swelling in any part of your body but especially your face, throat, and tongue

Seek immediate medical attention if you develop any of these symptoms after taking antibiotics. If you experience wheezing, throat, or facial swelling, call an ambulance. These symptoms could be life-threatening.

Allergy to penicillins is quite rare. If you’ve had sickness or a headache, this is a side-effect, not an allergic reaction. Of course, do tell your doctor, but you can probably safely take penicillin if they recommend it.

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 infection with that type of bacteria. This is more likely if we don’t use antibiotics properly – not finishing a complete course of treatment, for instance. These resistant bacteria can then cause a further infection or spread to other people.

Some bacteria can become resistant to several different antibiotics. 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 it’s resistant to most of the usual antibiotics your doctor would prescribe.

Using antibiotics when they’re not really needed increases the likelihood of resistant bacteria developing. 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

Bupa's medicines checklist (PDF, 0.8MB), helps you understand what to check for before taking a medicine.