What are the side effects of antibiotics?

Like all medicines, antibiotics that you may be taking for a bacterial respiratory tract infection have the potential to cause side effects.

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.

Diarrhoea, nausea and vomiting are common

The most common side effects of almost all antibiotics are stomach problems such as diarrhoea, nausea and vomiting. Between 1 and 10 in every 100 people taking an antibiotic will experience these side effects (up to 10%).

These side effects happen because antibiotics can sometimes cause the gut (intestine) lining to become inflamed, and may disturb the balance of ‘good’ (harmless) and ‘bad’ (potentially disease-causing) bacteria in the gut. These effects may result in your intestines being less able to absorb water and nutrients from food, resulting in diarrhoea.

Thrush (candidiasis)

Thrush is an infection caused by the yeast Candida albicans (or candida). Candida normally lives harmlessly in your body, including in the vagina. Its growth is kept under control by your immune system and the other bacteria found in the body. For some women, taking antibiotics can upset the normal balance of bacteria in the vagina and this allows candida to grow, causing thrush. The symptoms of thrush include itchiness, pain and a vaginal discharge.

Thrush can also affect other parts of the body including your mouth (white patches will be visible) and skin.

Thrush is common; between 1 and 10 in every 100 people who have recently been taking antibiotics will experience thrush (up to 10%).

You can buy antifungal medicines (e.g. Canesten) to treat thrush from your pharmacy without a prescription.

Clostridium difficile-associated disease

Clostridium difficile (C. difficile) is a bacterium found in the gut (intestines) of many people, together with many other types of bacteria (e.g. Escherichia coli or E. coli). All the bacteria generally live in balance together, with the harmless (‘good’) gut bacteria helping to keep the number of ‘bad’ C. difficile bacteria low.

If you are taking an antibiotic (e.g. amoxycillin) that kills the bacteria causing your respiratory tract infection and some of the ‘good’ bacteria in your gut, but not C. difficile, this allows the C. difficile bacteria to thrive and multiply and potentially cause C. difficile-associated disease.

This doesn’t happen often; between 1 and 10 in every 1000 people who have recently been taking antibiotics will experience C. difficile-associated disease (0.1%–1%).

Symptoms of a C. difficile infection can range from mild diarrhoea to life-threatening bowel inflammation. No treatment is needed in most mild C. difficile infections, but antibiotics may be needed to treat more severe infections.

Allergic reactions

Some antibiotics (e.g. penicillin or a related antibiotic such as amoxycillin) can cause allergic or hypersensitivity reactions such as hives (large, red, raised areas on the skin), fever and breathing problems. Very rarely*, a person may experience a severe or immediate allergic reaction to the antibiotic (anaphylaxis).

It is not known why this happens to some people and not others, but it may mean that you will have to avoid taking amoxycillin and any penicillin-related antibiotic in the future.

If you have developed a rash or hives, or you have had a severe or immediate allergic reaction to an antibiotic in the past, make sure you tell your doctor so you don’t get the same kind of antibiotic prescribed again. Another type of antibiotic will probably not cause the same problem.

*Rare: fewer than 1 in 1000 people will experience the side effect.

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).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).

Antibiotic resistance

While not a direct side effect of antibiotics, antibiotic resistance can be another unintended effect of taking antibiotics. If you take an antibiotic when you don’t need one, or you don’t take the antibiotic as prescribed by your doctor, this can lead to antibiotic resistance.

Find out what antibiotic resistance is, how it can affect you, and how you can help prevent it.

Find out more

Read our page on understanding the side effects of your medicine.

Phone for medicines information

Call NPS Medicines Line on 1300 MEDICINE (1300 633 424) to get information about your prescription, over-the-counter and complementary medicines (herbal, ‘natural’, vitamins and mineral supplements) from a pharmacist. Your call will be answered by healthdirect Australia (except Queensland and Victoria).
  • Rossi S, ed. Australian Medicines Handbook [online]. Adelaide: AMH, July 2012.