Understanding drug interactions

There may be times when one medicine can mix badly with another in your body. This can change how well the medicines work or their side effects. This is known as an interaction. Interactions also happen when medicines mix with certain foods or drinks (including alcohol). Find out what you can do to avoid interactions.

 
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What are interactions, and when do they happen?

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Interactions happen when another medicine, food or drink (including alcohol) changes how a medicine works, or changes its side effects. Some of these interactions can be severe.

There are many types of interactions. The most common are:

  • making your body absorb or get rid of a medicine slower or faster than normal. Your normal dose is either too strong or not strong enough
  • adding two or more medicines that do similar things in your body. This makes the combined effect much stronger than you need.

Interactions are most likely to cause problems when you:

  • start taking a medicine
  • stop taking a medicine
  • increase the dose of a medicine.

Ask your health professional if there are any likely interactions you should know about. This is vital when you are starting a new medicine. You may need to watch out for unexpected symptoms in the first few days after your medicines change in any way.

Looking for your medicine's side effects or interactions?

Use the Medicine Finder to search for your medicine and its active ingredient. You can find out about its side effects and interactions. You can also ask your doctor or pharmacist.

Interactions between different medicines

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Many, but not all, medicines can interact with each other. Some interactions between medicines are well known. For others, it can be hard to predict whether this will affect the certain person taking the medicines. The more medicines you are taking, the more likely interactions are. 

Older people, people with chronic illnesses and young children are more likely to have interactions. Their bodies may not handle medicines as well as other people’s. But interactions can happen to anybody who takes more than one medicine.

Always tell your health professional about any other medicines you are using. This can also be complementary or alternative medicines, bush medicines and supplements. Keeping a Medicines List will help you keep track of all the medicines you are taking. If you have a smartphone, you can use the MedicineWise app to track your medicines.

All prescription medicines have a consumer medicine information leaflet (CMI). You can get from your pharmacist or read online via our Medicine Finder. See the ‘Taking other medicines’ section. You will find a list of known interactions with other medicines.

Find your medicine in the Medicine Finder


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Ask your pharmacist for advice on medication organisers, such as blister packs. It can help separate your medicines and make it easier to remember when to take them.

Interactions between medicines and food

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You should not take some medicines with meals or with certain foods and drinks. This is because the medicine and the food interact in some way.

All medicines have instructions about when you should take them. Keep these in mind. All medicines are different. 

Talk to your health professional when they prescribe you a new medicine. They will give you any special instructions about food or drink. Check with your health professional for any instructions on the medicine’s label. Read the CMI leaflet in the ‘How to take it’ section.

You can also call the NPS Medicines Line on 1300 MEDICINE (1300 633 424). A pharmacist will provide you information about interactions with your prescription, over-the-counter (non-prescription) and complementary medicines (herbal, ‘natural’, vitamins and minerals).

Examples of instructions about food with medicines

Instruction Example
Take on an empty stomach Some antibiotics, such as flucloxacillin, don't work as well if taken with food. You may find the medicine is causing an upset stomach when taken without food. Talk to your health professional. There may be other ways you can take it to improve the problem.
Take with meals You should take many common arthritis medicines and pain relievers with meals. This includes aspirin, ibuprofen and naproxen. This helps protect against stomach side effects, such as an upset stomach. This does not apply to all medicines. Check the instructions for your certain medicine.
Don't take with certain foods Take care when taking some antibiotics, such as doxycycline. This also applies for some osteoporosis medicines, such as alendronate. Avoid taking them at the same time as calcium-rich foods. They don’t work as well. This includes milk, cheeses and yoghurt, and calcium supplements.

Examples of warning labels on medicines to look out for

Interactions between medicines and alcohol

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Alcohol interacts with many medicines. This includes some prescription, pharmacy and complementary medicines.

The effects of combining alcohol and medicine depend on:

  • the type and dose of the medicine you are taking
  • the amount of alcohol you drink
  • personal factors, such as your genes, gender and other health conditions.

In most cases, women and older people are more likely to have this kind of interaction. This is because alcohol affects them more.

It takes many hours for your body to process and remove alcohol. Interactions between alcohol and medicines can occur at any time you have a large amount of alcohol in your body. It is not just when you take medicines and drink alcohol at the exact same time.

Always check your medicine’s label. Avoid alcohol if it gives you a warning. You may be unsure about drinking alcohol while taking a medicine. Ask your health professional for advice.

Examples of warning labels on medicines to look out for


When alcohol doesn’t mix well with medicines

Medicine typeInteraction with alcohol
Sleeping, anti-anxiety and antidepressant medicines Alcohol can increase the effects of medicines that relax or sedate the body. These medicines may make you feel more drowsy or dizzy. This may make it harder for you to think clearly. You are more likely to fall and impair the way you do complex things like drive a car.
Cough, cold, allergy and travel sickness medicines You may need to buy cough, cold, allergy and travel sickness medicines from pharmacies that often contain ingredients that relax or sedate you. These ingredients may interact with alcohol. It can cause you to feel more drowsy or dizzy.
Pain relievers Some common pain relievers can interact with alcohol. These include aspirin, celecoxib, ibuprofen and naproxen. They can cause stomach upsets, stomach bleeding and ulcers.

In most cases, an alcoholic drink or two now and then is not likely to cause problems. Often having more than three drinks a day may increase your risk of stomach problems with these medicines.

Interactions between medicines and grapefruit

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Chemicals in grapefruit can affect how medicines work


Furanocoumarins are chemicals in grapefruit. They can affect the way your body passes many medicines. This may occur in the gut or liver before they reach your bloodstream. 

This happens because of the effect of these grapefruit chemicals on an enzyme called  'CYP3A4'. These enzymes help metabolise (break down and clear) many medicines. Grapefruit can affect medicines taken by mouth (as tablets or liquids) via your gut. When this happens, more of the medicine may get into your body. They may work too strongly or cause side effects you do not want.

The whole grapefruit contains furanocoumarins, the chemicals that can interact with medicines. This includes the juice and peel. For this reason, your doctor may advise you to not to eat grapefruit or drink grapefruit juice if you are taking a medicine that interacts with it. They may also advise you to avoid other related fruits that could interact with medicines.

Some other citrus fruits might have a similar effect

Limes and bitter oranges (eg, Seville oranges) may also cause interactions with medicines. Other citrus fruits such as sweet oranges and lemons don't have this same effect.

Some medicines are already known to interact with grapefruit

We know some medicines interact, or likely interact with grapefruit. This includes prescription medicines, over-the-counter (non-prescription) or complementary medicines (herbal, 'natural', vitamin and mineral supplements).

Some types of medicines that interact with grapefruit are:

  • some statin medicines to lower cholesterol, such as simvastatin and atorvastatin
  • some medicines that treat high blood pressure, such as nifedipine
  • some organ transplant-rejection medicines, such as cyclosporine
  • some anti-anxiety medicines, such as buspirone (not widely available in Australia)
  • some corticosteroids that treat Crohn disease or ulcerative colitis, such as budesonide
  • some medicines that treat abnormal heart rhythms, such as amiodarone
  • some antihistamines, such as fexofenadine.

Examples of warning labels on medicines to look out for

If you are concerned about medicine interactions

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You may have concerns about whether any of your medicines interact with foods or other medicines you take. Talk to your doctor or pharmacist, or read the 'Taking other medicines' section of the consumer medicine information (CMI) leaflet. You can download the CMI for your medicine from the NPS MedicineWise Medicine Finder. You can also ask your pharmacist or doctor to print it out for you.

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Get advice from your doctor or pharmacist if you're taking a medicine that may interact with foods in your diet. Your doctor or pharmacist can discuss your risk of an interaction with you. They can suggest an alternative treatment if needed.

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Call the NPS Medicines Line on 1300 MEDICINE (1300 633 424). A pharmacist will give you information about interactions with your prescription, over-the-counter (non-prescription) and complementary medicines (herbal, 'natural', vitamins and minerals).

You may you suspect that your medicine is causing a problem and you're worried about using it. Call the Adverse Medicines Events (AME) Line on 1300 134 237. AME line provides consumers with an option to report and discuss adverse experiences with medicines.