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

There are many types of antibiotics. Not all of them work in the same way. For instance, penicillin destroys bacterial cell walls. Other antibiotics can affect the way the cell works.

Doctors choose an antibiotic based on the bacteria that often cause a certain infection. Sometimes your doctor will do a test. This is to identify the exact type of bacteria causing your infection. It also checks how it reacts to certain antibiotics.

Know the active ingredient

The active ingredient is the chemical in a medicine that makes it work. Antibiotic medicines may have one or more active ingredients. They can come in one or more brand names. The label should tell you the active ingredient and the brand name.

You can find out more about the active ingredient. Read the Consumer Medicine Information (CMI) for your brand of medicine. You can find it on our Medicine Finder page or ask your pharmacist or doctor.

Find out more about how to read CMIs

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

• Stomach problems. This can be having loose bowels (diarrhoea), having the urge to vomit and vomiting. 
• Thrush infections. This can affect the mouth (you will see white patches). In some people, this can also occur in the vagina (causing an itch, pain and discharge).

Other less common side effects include:

• loose bowel movement (diarrhoea) that persists. A gut infection can cause this. This may be severe. It may need further checks and treatment
• allergic reactions, such as hives (large, red, raised regions on the skin), fever and breathing problems.

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

The CMI for your medicine also lists the most common side effects. It also tells you if it affects how other medicines work.

Who can I ask about side effects?

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

You may have questions about your medicines or want to learn more about side effects can also call the Medicines Line on 1300 633 424 (Monday to Friday, 9am-5pm AEST). 

You may need to report likely side effects. Call the Adverse Medicine Events (AME) line on 1300 134 237 no matter where you are in Australia (Monday to Friday, 9am–5pm AEST).

Find out more about medicine side effects and interactions.

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.

You may be healthy and well most of the time. This means your immune system will take care of most chest and lung infections by itself. This includes both viral and some bacterial infections.

People who are more likely to need antibiotics:

• have severe infections caused by bacteria (eg, whooping cough)
• have a long-term health condition (eg, asthma, diabetes or lung disease)
• are older or often in poor health, or have a weakened immune system (eg, due to HIV infection)
• have a higher risk of complications with a chest or lung infection (complications often include bacterial infections).

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

Be medicinewise with antibiotics

• Take your antibiotics at the right time. Take them for as long as the doctor tells you to.
• Don’t take more than the dose your doctor has prescribed. Sometimes the medicine box may contain more pills than what your doctor tells you to take.
• Don’t keep unused antibiotics for a later time. Return pills you have left over to your pharmacy to dispose.
• Don’t share antibiotics with others. They may not have the same infection and this can lead to antibiotic resistance.

The Consumer Medicine Information (CMI) for your medicine lists useful information. This includes how to take your antibiotic (eg, with food) and what to do if you miss a dose. The CMI will list the normal dose. Sometimes your doctor will prescribe a different dose that is more suited for you.

Alcohol and antibiotics

If you drink alcohol in moderation, it will not likely be a problem with most antibiotics. There may be times when this is not the case. Avoid drinking alcohol if you are taking these antibiotics:

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

Contraceptives and antibiotics

Some antibiotics can affect how well some hormonal contraceptives work. This is the case with rifampicin (Rifadin, Rimycin) and rifabutin (Mycobutin). These antibiotics are usually only used for:

• treating severe bacterial infections like tuberculosis and methicillin-resistant Staphylococcus aureus (MRSA)
• preventing meningitis (an infection around the brain or spinal cord).

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

These changes allow the bacteria to survive or 'resist' the antibiotic. The antibiotic no longer works to kill the bacteria or stop them from spreading. The bacteria are now resistant to the antibiotic.

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 people use antibiotics, 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. Over time, medicines may stop working against many common infections.

The World Health Organization (WHO) has issued a warning. They say 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 many types of antibiotics. These bacteria are now very hard to treat because of antibiotic resistance:

• the methicillin-resistant Staphylococcus aureus bacteria (golden staph) is common in hospitals.
• the bacteria that causes tuberculosis (Mycobacterium tuberculosis)
• strains of Escherichia coli, the bacteria that causes many urinary tract infections.

If you have an antibiotic-resistant bacterial infection:

• you will have it for longer
• you may be more likely to have complications from the infection
• you could remain infectious for longer. You may pass your infection to other people. This can worsen the problem.

You could be passing it on

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

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

• Know that viruses cause colds and flu. Antibiotics treat bacterial infections, not viruses.
• Tell your doctor you only want an antibiotic if you really need it.
• 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. Ask your friends and family to as well.

Find out more about antibiotic resistance

There are five things you can pledge to do to reduce antibiotic resistance:

1. I will not ask for antibiotics for colds and the flu. They have no effect on viruses.
2. I understand that antibiotics will not help me to get better faster from a viral infection.
3. I will only take antibiotics in the way my doctor prescribes them.
4. I understand that it is likely to pass on antibiotic-resistant bacteria to others.
5. I will make a greater effort to stop the spread of germs. I will practise good hygiene.