Antibiotics work by blocking vital processes in bacteria, killing the bacteria or stopping them from multiplying. This helps the body's natural immune system to fight the bacterial infection. Different antibiotics work against different types of bacteria.

• Antibiotics that affect a wide range of bacteria are called broad spectrum antibiotics (eg, amoxicillin and gentamicin).
  
• Antibiotics that affect only a few types of bacteria are called narrow spectrum antibiotics (eg, penicillin).
  

Different types of antibiotics work in different ways. For example, penicillin destroys bacterial cell walls, while other antibiotics can affect the way the bacterial cell works.

Doctors choose an antibiotic according to the bacteria that usually cause a particular infection. Sometimes your doctor will do a test to identify the exact type of bacteria causing your infection and its sensitivity to particular antibiotics.

Know the active ingredient

Antibiotic medicines may contain one or more active ingredients and be available under different brand names. The active ingredient is the chemical in a medicine that makes it work. The medicine label should tell you the active ingredient and the brand name.

To find out more about the active ingredient see the Consumer Medicine Information (CMI) for your brand of medicine, available on our Medicine Finder page or from your pharmacist or doctor.

Find out more about how to read CMIs

Like all medicines, antibiotics have the potential to cause side effects. When antibiotics are necessary, the benefits far outweigh the risks, but when they are not needed, you are taking an unnecessary risk.

Up to 10% of people taking an antibiotic may experience these common side effects:

• stomach problems like diarrhoea, nausea and vomiting 
  
• thrush infections, which can affect the mouth (white patches will be visible) and in women can also occur in the vagina (causing itchiness, pain and discharge).
  

Other less common side effects include:

• ongoing diarrhoea caused by an intestinal infection, which may be serious and require further investigation and treatment
  
• allergic reactions, such as hives (large, red, raised areas on the skin), fever and breathing problems.
  

Ask your doctor or pharmacist about the possible side effects of your medicine. You should also ask if there are any medicines you should not take with your antibiotic.

The Consumer Medicine Information (CMI) for your medicine also lists the most common side effects as well as any interactions with other medicines.

Who can I ask about side effects?

If you’re concerned that you or someone in your care may have had side effects related to a medicine, seek medical advice.

People with questions about their medicines or seeking general information about side effects can also call the NPS Medicines Line on 1300 633 424 (Monday-Friday, 9am-5pm AEST). 

To report possible side effects call the Adverse Medicine Events (AME) line on 1300 134 237 from anywhere in Australia (Monday–Friday, 9am–5pm AEST).

Find out more about medicine side effects and interactions

Good-quality, reliable clinical studies have shown that antibiotics do not improve the symptoms of a cold or the flu. This is because antibiotics work only on infections caused by bacteria – common colds and the flu are infections caused by viruses.

Antibiotics will not 

• help a cold or the flu get better faster
  
• stop a cold or the flu from getting worse or 
  
• stop a cold or the flu from spreading to other people.
  

If you are usually healthy and well, your immune system will take care of most respiratory tract infections – both viral and some bacterial infections – by itself.

However, antibiotics are more likely to be needed for people who:

• have serious infections caused by bacteria (eg, whooping cough)
  
• have an ongoing health condition (eg, asthma, diabetes or lung disease)
  
• are older or in generally poor health, or have a weakened immune system (eg, due to HIV infection)
  
• have a higher risk of complications with a respiratory tract infection (complications often include bacterial infections).
  

Using antibiotics when you don't need them can contribute to the problem of antibiotic resistance. This might mean that if you have a serious infection, such as pneumonia, in the future, antibiotics may not work as well.

If you are prescribed antibiotics, it is important that you follow your doctor’s advice on when, how and for how long to take them.

Be medicinewise with antibiotics

• Take your antibiotics at the right time, and for as long as directed.
  
• Don’t take more than the dose your doctor has prescribed – sometimes the medicine box may contain more antibiotic pills than you will be instructed to take.
  
• Don’t keep unused antibiotics for another time – return leftover pills to your pharmacy for disposal.
  
• Don’t share antibiotics with others – they may have a different infection and this can lead to antibiotic resistance.
  

The Consumer Medicine Information (CMI) for your medicine lists useful information including how to take your antibiotic (eg, with food) and what to do if you miss a dose. The CMI will list the usual dose, but sometimes your doctor will prescribe a different dose that is more suitable for you.

Alcohol and antibiotics

Drinking alcohol in moderation is unlikely to be a problem with most antibiotics but there are some exceptions. Avoid drinking alcohol if you are taking these antibiotics:

• metronidazole (brand names include: Flagyl, Metrogyl, Metronide)
  
• tinidazole (brand names: Fasigyn, Simplotan).
  

Contraceptives and antibiotics

Rifampicin (Rifadin, Rimycin) and rifabutin (Mycobutin) are the only antibiotics that are thought to affect how well some hormonal contraceptives work. These antibiotics are usually only used to treat serious bacterial infections like tuberculosis and methicillin-resistant Staphylococcus aureus (MRSA), and for preventing meningitis (an infection around the brain or spinal cord).

Ask your health professional for advice as you may need to use extra contraceptive precautions while you are taking rifampicin or rifabutin (and after finishing the prescribed course). 

Antibiotic resistance happens when bacteria change to protect themselves from an antibiotic. Bacteria become resistant when their genes mutate after being in contact with an antibiotic. These changes allow the bacteria to survive or 'resist' the antibiotic, so that the antibiotic no longer works to kill the bacteria or stop them from multiplying.

Unfortunately, bacteria can also develop antibiotic resistance through contact with other bacteria. Resistant bacteria can pass their genes to other bacteria, forming a new antibiotic-resistant 'strain' of the bacteria.

The more antibiotics are used, the more chances bacteria have to become resistant to them.

As more antibiotics stop working against bacterial infections, doctors will have fewer antibiotics to use. Many common infections may eventually become untreatable with medicines.

The World Health Organization (WHO) has warned that antibiotic resistance is one of the greatest threats to global public health today.

Find out more about antibiotic resistance

What are 'superbugs'?

'Superbugs' are bacteria that are resistant to several different antibiotics. The methicillin-resistant Staphylococcus aureus bacteria commonly found in hospitals, and the bacteria that cause tuberculosis (Mycobacterium tuberculosis), are now very hard to treat because of antibiotic resistance, as are strains of Escherichia coli, the bacteria that causes many urinary tract infections.

Using antibiotics when you don't need them may mean that they won't work for you when you do need them in the future.

If you have an antibiotic-resistant bacterial infection:

• you will have the infection for longer
  
• you may be more likely to have complications of the infection
  
• you could remain infectious for longer, and pass your infection to other people, which increases the problem.
  

You could be passing it on

You could be passing on antibiotic-resistant bacteria to other people:

• if you take antibiotics for cold and flu viruses
  
• if you don’t take antibiotics as prescribed 
  
• if you neglect good hygiene.
  

Australia has one of the highest rates of antibiotic consumption in the developed world, but the good news is that there are steps you can take to prevent the spread of antibiotic resistance.

• Understand that colds and flu are caused by viruses, and that antibiotics treat bacterial infections, not viruses
  
• Tell your doctor you only want an antibiotic if it is really necessary
  
• Take the right dose of your antibiotic at the right time, as prescribed by your doctor
  
• Take your antibiotic for as long as your doctor tells you to
  
• Take the pledge to fight antibiotic resistance and encourage your friends and family to as well.
  

Find out more about antibiotic resistance