• Unwanted side effects can be caused by all kinds of medicines, including prescription and over-the-counter medicines, and complementary medicines such as herbal preparations and vitamins.
  • If you are worried about any medicines you are taking, you should seek immediate medical attention.
  • Taking some complementary medicines and prescription medicines at the same time can increase the risk of side effects, because the active ingredients in the various preparations can ‘clash’.
  • Tell your doctor about all the medicines you are taking, including prescription, over-the-counter and complementary. You may like to have an annual review of all your medications.
A side-effect is an unwanted symptom caused by medical treatment. Side effects can be caused by all kinds of medicines, including prescription and over-the-counter medicines, complementary medicines including herbal preparations, vitamins, and some products dispensed by naturopaths and other practitioners of complementary medicine. It’s estimated that around 230,000 Australians are admitted to hospital every year because of problems associated with the use of medicines, including side effects. Death can also occur in severe cases. It is in your best interests to manage your medicines wisely. See your doctor or pharmacist for further information and advice.

Prescription medicines can cause side effects

All medicines can cause unwanted side effects. For example, antibiotics such as those in the sulfonamide and penicillin families cause allergic reactions in around five per cent of the population. Skin rashes are a common reaction. However, whether a reaction is caused by the medicine or the illness that it is used to treat is sometimes difficult to tell.

A further complication is the interaction of medicines with any other medicines the person may be taking, including prescription, over-the-counter and complementary medicines. . 

Complementary medicines also cause side effects 

About 60 per cent of Australians use complementary medicines at least once a year. Many people believe that alternative or complementary preparations, such as herbal remedies, are safer because they are derived from natural sources. This isn’t always true. 

Some herbs can act on the body as powerfully as any conventional medicine, and unwanted side effects can occur.

Some examples of complementary medicines that can cause side effects include: 

  • echinacea – more than 20 different types of reactions have been reported, including asthma attacks, hives, swelling, aching muscles and gastrointestinal upsets
  • feverfew – pregnant women are cautioned against using this herb, as it can trigger uterine contractions. In animal experiments, the use of feverfew was found to trigger spontaneous abortions (miscarriages)
  • asteraceae plants – (from the daisy family, including feverfew, echinacea, dandelion and chamomile) – side effects include allergic dermatitis and hay fever.

Complementary medicines can interact with prescription medicines

About 20 per cent of Australians are thought to take complementary medicines and prescription medicines at the same time. This increases the risk of side effects, because the active ingredients in the various preparations can interact in a negative way.

For example, the medicines may have similar active ingredients that act in the same way, or the complementary medicine may increase or reduce the effectiveness of the prescription medicine.

Some interactions between complementary medicines and prescription medicines that can put people’s health at risk include: 

  • Echinacea may interact with medications broken down by the liver. 
  • Many complementary medicines – including feverfew, ginkgo and chamomile – may increase the risk of bleeding in people taking anticoagulant medicines such as warfarin and anti-inflammatory medicines such as aspirin.
  • St John’s wort has caused breakthrough bleeding and could cause unwanted pregnancy in women taking the oral contraceptive pill.
  • St John’s wort increases serotonin. Taking St John’s wort at the same time as other medicines that increase serotonin (such as antidepressants) increases the risk of serotonin toxicity (serotonin syndrome) – symptoms include tremors, high temperature and low blood pressure and can range from mild to life threatening. 

Alcohol used with medicines can cause side effects

Consuming alcohol with some medicines can also cause unwanted and sometimes dangerous side effects. For example:

  • Alcohol can cause drowsiness or dizziness when taken with antihistamines, antidepressant medicines, sleeping tablets or medicines for anxiety.
  • Medicines for high blood pressure and travel sickness, and some pain relievers can also be affected by alcohol.
  • Some antibiotics interact negatively with alcohol. For example, the antibiotics metronidazole and tinidazole can cause a severe reaction if combined with alcohol, including nausea, vomiting, skin flushing, headache and a fast or irregular heartbeat. Other antibiotics can cause stomach upset, drowsiness or dizziness if combined with alcohol. Ask your doctor for advice about alcohol when you are prescribed antibiotics.

Alcohol can stay in your system for several hours after your last drink, so it is important to be aware that interactions can occur if you take your medication within this time frame.

Talk to your doctor or other health professional for advice about your medication and drinking alcohol.

What to do if you experience side effects

If you experience side effects when taking medication:

  • In an emergency, call triple zero (000).
  • Note the side effects and consult your doctor if you have any concerns. The dose or type of medicine may need to be adjusted. 
  • If you are sensitive to a particular medicine, and a substitute is not available, your doctor may suggest desensitisation therapy.
  • Call the NPS Medicines Line on 1300 633 424 or the Adverse Medicines Events Line on 1300 134 237 for advice. These phone-line services allow consumers to report or receive advice on side effects. They are not emergency services.

How to reduce the risk of side effects

To reduce your risk of experiencing side-effects:

  • Take all medicines strictly as prescribed. (Taking medication incorrectly can cause side effects.)
  • Don’t take anyone else’s medicines.
  • Learn about your medication. All prescription medicines have an information leaflet called Consumer Medicine Information (CMI). This includes detailed information on the medicine in plain English, including use, side effects and precautions. Your pharmacist can give you the CMI for your medicine.
  • Ask your pharmacist for advice if you buy over-the-counter medicines. They can advise you about side effects and interactions with other medicines you are taking. Be aware that medicines you buy in the supermarket can also cause side effects.
  • Tell your doctor about all the medicines you take, including prescription, over-the-counter and complementary medicines.
  • Ask your doctor if improving your lifestyle could reduce your need for medication. Some conditions can be better managed with changes to your diet and regular exercise. 
  • Have an annual review of all the medicines you take. This is particularly important for older people because, as people age, they are more likely to have side effects from medicines. Any medicines considered no longer necessary should be stopped. Ask your doctor if you might benefit from a Home Medicines Review. A pharmacist will review all the medicines you take. 
  • Return unwanted and out-of-date medicines to your pharmacy for safe disposal. This service is provided free of charge.
  • Talk to your pharmacist about dosage aids that can help you organise your pill taking. You may be at risk of making mistakes if you take many different medicines at different times.
  • Ask your doctor or pharmacist questions so you can clearly understand the benefits and risks of your medicines.

Where to get help

  • In an emergency, call triple zero (000).
  • Your doctor
  • NURSE-ON-CALL Tel. 1300 60 60 24 – for expert health information and advice (24 hours, 7 days)
  • Your pharmacist
  • Adverse Medicines Events (AME) Line Tel. 1300 134 237 – to report a problem with your medicine
  • Medicines Line Tel. 1300 MEDICINE (1300 633 424) – for information on prescription, over-the-counter and complementary medicines 
  • Victorian Poisons Information Centre Tel. 13 11 26 – seven days a week, 24 hours a day – for advice about poisonings, suspected poisonings, bites and stings, mistakes with medicines and poisoning prevention advice
  • Important interactions between St John’s Wort (Hypericum Perforatum) , 2001, Therapeutic Goods Administration. More information here.
  • Adverse reactions to alternative medicines, 2015, Australasian Society of Clinical Immunology and Allergy. More information here.
  • Allergic reaction to antibiotics, Australasian Society of Clinical Immunology and Allergy. More information here.
  • Segal, R. and Pilote, L. (2006), ‘Warfarin interaction with Matricaria chamomilla’, in Canadian Medical Association Journal, vol. 174, no. 9, pp. 1281–2. More information here.

More information


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Last updated: February 2017

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