Antibiotics attack bacteria, which are germs responsible for many different infections. There are many different bacteria and many different antibiotics. Each antibiotic attacks different types of bacteria and will be useful for treating particular infections. Some are broad spectrum, affecting a wide range of bacteria – and some are narrow spectrum, affecting only a few types.
Antibiotics can be life-saving in the case of serious infections, but they can also have harmful side effects. Overuse of antibiotics can lead to antibiotic resistance, making them less effective in the future.
Antibiotics don’t kill viruses
Not all infections are due to bacteria. Many, such as the common cold, flu, glandular fever and chickenpox, are due to viruses. Antibiotics do not affect viruses and are of no use in treating viral infections. Most simple coughs and colds do not need antibiotic treatment.
It is sometimes not possible to be certain if an infection has a bacterial or viral cause. In these circumstances, your doctor’s experience and findings after examining you, and possibly testing for the cause of infection, will determine if antibiotics are needed.
Common infections and antibiotics
Many infections will clear up by themselves without the need for antibiotics. And many infections are caused by viruses – so antibiotics won’t work. Your doctor weighs these things up when they decide whether or not to prescribe antibiotics for your infection.
You may wonder why not prescribe anyway, ‘just in case’? But there are consequences to inappropriate prescribing of antibiotics:
- bacteria develop resistance to antibiotics, making the drugs ineffective
- you may suffer an adverse reaction to the antibiotics.
Here are some common everyday infections – and whether antibiotics will help treat them.
Middle ear infection (otitis media) – This is one of the most common reasons for antibiotic prescriptions in children. However, most children do not need antibiotics, and only need painkillers, but some such as very young children, or children with infection in both ears, may need antibiotics.
Strep throat and tonsillitis – Most cases of sore throat or tonsillitis are caused by viruses; therefore antibiotics will not work. Even if the cause is suspected to be bacterial, antibiotics will not shorten the course of the illness by very much (less than a day).
Chlamydia – Chlamydia is a sexually transmitted infection caused by a bacterium. It can cause complications, such as pelvic inflammatory disease or infertility, if not treated. For that reason, Chlamydia is treated with antibiotics.
Conjunctivitis – This inflammation of the membrane lining the eyelid may be caused by viruses, bacteria or an allergic reaction. Bacterial conjunctivitis is usually mild and clears up on its own within a few days, but it is highly contagious. Sometimes your doctor may prescribe antibacterial eye drops or ointment.
Urinary tract infection (UTI) – UTIs are usually caused by bacteria and are often treated with a course of antibiotics. Women who have recurrent UTIs may be given a prescription that they can fill themselves when they next have an infection.
Skin infections – Whether bacterial skin infections should be treated with antibiotics depends on the type of bacteria causing the infection and how extensive or involved the infection is. Bacterial skin infections include impetigo, boils, folliculitis and cellulitis. Some mild infections of impetigo or folliculitis can be treated with antibiotic creams or ointment, e.g. mupirocin (Bactroban). Other infections may need antibiotic tablets or intravenous antibiotics.
Acne – Although acne is not an infection, antibiotics may be used for an anti-inflammatory effect if the acne is moderate to severe. This is usually for a few months only.
Respiratory tract infections – Antibiotics are widely prescribed for new respiratory tract infections, but they are often not appropriate and not recommended by guidelines. Antibiotics used to be the routine treatment, but doctors now know that the benefits are outweighed by the harm done, in terms of adverse reactions, costs and antibiotic resistance.
Some respiratory infections do need antibiotics, but they are not recommended for bronchitis or influenza. Antibiotics are indicated for some people with sore throat and a minority of people with rhinosinusitis (inflammation of the sinus and nasal cavities).
Do I need a prescription for antibiotics?
In Australia, antibiotics need to be prescribed by a doctor. But in Queensland, there are some very specific uses (antibiotics for urinary tract infections), where a pharmacist may dispense antibiotics without a prescription.
Do I have to finish the course or not?
When you are prescribed a course of antibiotics, it is important to take the full course as directed by your doctor, even if you start feeling better after a day or 2. This way, the bacteria will be fully exterminated, and the risk of recurrence reduced. If you stop before the course is finished, there is a risk of the bacteria becoming resistant to the antibiotic.
Antibiotics can take the form of tablets, capsules, creams, injections or be given intravenously. Some antibiotics should be taken with food, and others need to be taken on an empty stomach. Always check the instructions on the pack or ask your pharmacist.
What if I miss a dose?
Ideally, your doses of antibiotics should be as evenly spaced out as possible to maintain the concentration of drug in your body. If you miss a dose of your antibiotics or accidentally take 2 doses, your pharmacist will be able to advise what you should do, or the Consumer Medicines Information leaflet in the pack may have instructions.
Although antibiotics are generally very safe medicines, side-effects can sometimes occur. These are usually mild. The side effects that may be experienced with the antibiotic you are taking will be explained in the Consumer Medicines Information leaflet in the pack. If you have any concerns about side effects, talk to your pharmacist or doctor.
Common side effects of antibiotics include:
- mild stomach upset
- vaginal thrush in women, and
If you experience symptoms such as vomiting, rash, hives, shortness of breath, oral thrush, or severe watery diarrhoea, you should let your healthcare professional know.
Do antibiotics affect the oral contraceptive pill?
Some antibiotics, such as rifampicin, may affect the oral contraceptive pill, making it less effective. The Consumer Medicines Information leaflet that comes with your antibiotics will have a warning if this is the case, or your pharmacist or doctor will be able to advise.
You may need to use additional contraception, such as condoms, while this is the case or use a different method of contraception.
If the antibiotics have caused you to vomit or have diarrhoea, as a side effect, then your contraceptive pill may not have been absorbed by your body and won’t be effective at protecting you from getting pregnant.
Some people are allergic to certain antibiotics. This can result in a rash such as hives, and occasionally swelling of the lips or tongue developing soon after taking the antibiotic (usually within one hour). Very rarely, an allergic reaction to antibiotics results in a severe reaction with collapse and breathing difficulties – anaphylaxis.
Rashes that appear after taking an antibiotic are not always due to allergy – they may be due to the illness itself. If you develop a rash after taking an antibiotic, talk to your doctor. If any reaction is rapid and severe, call 000 for an ambulance.
Penicillin allergy can cause rash, itching and hives, as well as more severe reactions, including anaphylaxis. These reactions normally happen within an hour of taking the drug. But also, like other antibiotics, penicillin can cause side effects that aren’t allergic reactions.
Antibiotics and alcohol
Drinking alcohol when you’re unwell or taking medication is not generally a good idea. But, in addition, alcohol can cause a serious dangerous reaction if you drink it when you are taking certain antibiotics. Always read the warnings on your medicine label.
Alcohol must not be used when you are taking some medicines, including:
- metronidazole (brand names include Flagyl, Metrogyl, Metronide) or
- tinidazole (brand names Fasigyn, Simplotan).
With other antibiotics, whilst alcohol might not make them less effective, it will increase your chances of having side effects.
Are antibiotics safe when I’m pregnant or breastfeeding?
If you are pregnant, or you might be, or you are breastfeeding, you should let your doctor know, so they can take that into account when prescribing antibiotics. Many antibiotics are safe to use during pregnancy, but others are not suitable.
Unfortunately, bacteria can develop resistance to antibiotics. That means that the antibiotic will no longer kill the bacteria, and can’t fight the infection. This antibiotic resistance can develop if antibiotics are not used properly (misuse), or when they are used unnecessarily (overuse).
Increasing resistance to antibiotics is a significant problem and a major threat to public health, particularly with some bacteria that are capable of causing serious disease.
Australia overuses antibiotics, and we have antibiotic resistance here now. Almost half of Australians were given a prescription for antibiotics in 2014, which put Australia in 8th place for the highest prescribing of antibiotics in the OECD.
The more antibiotics are used, the greater the chance of resistant strains emerging, hence the need to be very selective about the use of antibiotics.
With antibiotic resistance, strains of bacteria may become resistant to many types of antibiotics – these are known colloquially as ‘superbugs’. Superbugs cause infections that are hard to treat.
MRSA (methicillin-resistant Staphylococcus aureus) is an example of a superbug. It is a strain of Staphylococcus aureus that has become resistant to multiple antibiotics and is very hard to treat. It is still sensitive to some antibiotics, but the options have decreased. MRSA is now circulating in the community in Australia.
Vancomycin-resistant Enterococci (VRE) – are Enterococci that are resistant to the antibiotic vancomycin. Enterococci are bacteria that live in the gut – they are part of the normal flora of humans and animals. Enterococci are usually harmless in healthy people, but they can cause infections such as endocarditis and urinary tract infections.
Vancomycin is an antibiotic which is often used to treat serious infections caused by bacteria resistant to other antibiotics. When Enterococci that are resistant to vancomycin cause infections, the resulting infections are very difficult to treat.
Carbapenem-resistant Enterobacteriaceae (CRE). The Enterobacteriaceae are another family of bacteria that live normally without problem in the human bowel. But sometimes, these bacteria may spread outside of the gut and cause infections, such as pneumonia or a urinary tract infection.
Carbapenems are a type of antibiotic used to treat serious infections. Some Enterobacteriaceae have acquired resistance to carbapenems, making any infections they cause very difficult to treat. There are still some antibiotics that can be used to treat infections caused by CRE.
Clostridium difficile is a type of bacterium. It lives in the digestive tract and can exist there without causing any problems. Healthy people can have Clostridium difficile (C. diff for short) in their intestine. But, if the local conditions in the intestine should change, Clostridium difficile can multiply out of control, and the toxins it produces can cause diarrhoea and inflammation of the colon (colitis). Severe inflammation of the colon can lead to death.
C. diff is treated with antibiotics, but it is difficult to treat. Antibiotics also kill off the good protective bacteria in the gut, making it easier for C. diff to take over.
Main types of antibiotics
Antibiotics can be grouped according to their class. Antibiotics within a class will share similar chemical and pharmacological properties, acting in the same way to kill or slow down the growth of bacteria. These are the common classes of antibiotics in Australia.
Aminoglycosides, e.g. amikacin, gentamicin, tobramycin.
Carbapenems, e.g. ertapenem, imipenem, meropenem.
Cephalosporins, e.g. cefaclor, cefalexin, cefalotin, cefazolin, cefepime, cefotazime, cefoxitin, ceftaroline, ceftazidime, ceftolozane, ceftriaxone, cefuroxime.
Macrolides, e.g. azithromycin, clarithromycin, erythromycin, roxithromycin.
Penicillins, e.g. amoxicillin, ampicillin, benzylpenicillin, dicloxacillin, flucloxacillin, phenoxymethylpenicillin, piperacillin, ticarcillin.
Quinolones, e.g. ciprofloxacin, moxifloxacin, norfloxacin.
Sulfonamides, e.g. sulfamethoxazole.
Tetracyclines, e.g. doxycycline, minocycline, tigecycline.
Other antibiotics, e.g. aztreonam, chloramphenicol, clindamycin, colistimethate, colistin, daptomycin, fidaxomicin, fosfomycin, lincomycin, linezolid, metronidazole, nitrofurantoin, pyrimethamine, sodium fusidate, teicoplanin, tinidazole, trimethoprim, vancomycin.
Last Reviewed: 03/03/2020
1. Australian Medicines Handbook. Antibacterials. Feb 2020 edition. https://amhonline.amh.net.au/chapters/anti-infectives/antibacterials
2. Therapeutic Guidelines. Antibiotic prescribing in primary care: Therapeutic Guidelines summary table 2019. https://tgldcdp.tg.org.au/etgAccess
3. eTherapeutic Guidelines. Sore throat. Is antibiotic therapy indicated? Published April 2019. Amended June 2019. © Therapeutic Guidelines Ltd (eTG December 2019 edition). https://tgldcdp.tg.org.au/viewTopic?topicfile=sore-throat&guidelineName=Antibiotic#toc_d1e379
4. eTherapeutic Guidelines. Otitis media. Published April 2019. Amended June 2019. © Therapeutic Guidelines Ltd (eTG December 2019 edition). https://tgldcdp.tg.org.au/viewTopic?topicfile=otitis-media&guidelineName=Antibiotic#toc_dle346
5. Sukumaran V, Senanayake S. Bacterial skin and soft tissue infections. Aust Prescr 2016; 39: 159-63. https://www.nps.org.au/australian-prescriber/articles/bacterial-skin-and-soft-tissue-infections
6. eTherapeutic Guidelines. Acne. Published November 2015. © Therapeutic Guidelines Ltd (eTG December 2019 edition). https://tgldcdp.tg.org.au/viewTopic?topicfile=acne&guidelineName=Dermatology&topicNavigation=navigateTopic#toc_d1e356
7. Amanda R McCullough, Allan J Pollack, Malene Plejdrup Hansen, et al. Antibiotics for acute respiratory infections in general practice: comparison of prescribing rates with guideline recommendations. Med J Aust 2017; 207 (2): 65-69. || doi: 10.5694/mja16.01042. https://www.mja.com.au/journal/2017/207/2/antibiotics-acute-respiratory-infections-general-practice-comparison-prescribing
8. Australian Medicines Handbook. Vancomycin. Feb 2020 edition. https://amhonline.amh.net.au/chapters/anti-infectives/antibacterials/glycopeptides/vancomycin
Antibiotics and children
Antibiotics: what are they and when does my child need them? Also, how to use antibiotics correctly and side effects of antibiotics.
Sore throat treatments
Find out about sore throat treatments including self-help measures, painkillers, lozenges, and when antibiotics are needed.
Colds: commonsense not antibiotics
If you have a cold, commonsense self-care measures and over-the-counter medicines can help to relieve symptoms. Antibiotics are not helpful in treating the common cold, which is caused by a virus.
Otitis media in children
Otitis media (middle ear infection) is a common childhood illness causing earache and fever. It usually gets better quickly with pain relievers but sometimes antibiotics are needed.
Meningitis in children
Meningitis means inflammation of the meninges - the lining around the brain and spinal cord. It is usually caused by a bacterial or viral infection.