Understanding drug interactions

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


What are interactions, and when do they happen?

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

There are many types of interactions, but the most common are:

  • making your body absorb or get rid of a medicine slower or faster than usual, so that your usual dose is either too strong or not strong enough
  • adding together two or more medicines that do similar things in your body, so that the combined effect is 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 when you are starting a new medicine, and 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 and find out about its side effects and interactions, or ask your doctor or pharmacist.

Interactions between different medicines

Many, but not all, medicines can interact with each other. Some interactions between medicines are well known, but it can be difficult to predict whether a specific person taking the medicines will be affected. The more medicines you are taking, the more likely interactions are. 

Older people, people with chronic illnesses and young children are more likely to experience interactions because their bodies do not handle medicines as well as other people’s. But interactions can happen to anybody who takes a combination of medicines.

Always tell your health professional about any other medicines you are using, including complementary or alternative medicines, bush medicines and supplements. Keeping a Medicines List will help you remember all the medicines you are taking. 

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

Find your medicine in the Medicine Finder

Interactions between medicines and food

Some medicines should not be taken with meals or with certain foods and drinks, because the medicine and the food interact in some way.

All medicines have instructions about when you should take them. Pay attention to these because all medicines are different. 

Every time you are prescribed a new medicine, ask your health professional if there are any special instructions about food or drink, and check for any instructions on the medicine’s label and in its CMI leaflet in the ‘How to take it’ section.

You can also call the NPS Medicines Line on 1300 MEDICINE (1300 633 424) to get information from a pharmacist about interactions with your prescription, over-the-counter 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. If you find the medicine is causing an upset stomach when taken without food, talk to your health professional. There may be another way you can take it to improve the problem.
Take with meals Many common arthritis medicines and pain relievers, such as aspirin, ibuprofen and naproxen, should be taken with meals. This helps protect against stomach side effects, such as indigestion. This does not apply to all medicines, so check the instructions for your particular medicine
Don't take with certain foods Some antibiotics, such as doxycycline, and some osteoporosis medicines, such as alendronate, don't work as well when taken at the same time as calcium-rich foods, such as milk and yoghurt, and calcium supplements.

Interactions between medicines and alcohol

Alcohol interacts with many medicines, including 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, and also on personal factors, such as genetics, gender and other health conditions. In general, women and older people are more likely to experience this kind of interaction, because alcohol affects them more.

It takes several hours for your body to process and remove alcohol. Interactions between alcohol and medicines can occur at any time that you have a significant amount of alcohol in your body, not just when you take medicines and drink alcohol at exactly the same time.

Always check your medicine’s label, and avoid alcohol if this warning is given. If you are unsure about drinking alcohol while taking a medicine, ask your health professional for advice.

When alcohol doesn’t mix well with medicines

Sleeping, anti-anxiety and antidepressant medicines 

Alcohol can increase the effects of medicines that relax or sedate the body, such as sleeping tablets, anti-anxiety medicines and antidepressant medicines. Increased drowsiness and dizziness may make it harder for you to think clearly, make you more likely to fall and and impair your ability to do complicated things like drive a car.

Cough, cold, allergy and travel sickness medicines 

Cough, cold, allergy and travel sickness medicines bought from pharmacies often contain ingredients that relax or sedate you. These ingredients may interact with alcohol to cause increased drowsiness and dizziness.

Pain relievers 

Some common pain relievers, such as aspirin, celecoxib, ibuprofen and naproxen, can interact with alcohol to cause stomach upsets, stomach bleeding and ulcers.

In general, the occasional alcoholic drink or two is unlikely to cause problems, but regularly having more than three alcoholic drinks a day may increase your risk of stomach problems with these medicines.

Interactions between medicines and grapefruit

Chemicals in grapefruit can affect how medicines work

It has been known for some time that certain chemicals in grapefruit (known as furanocoumarins) can interfere with the way your body processes or metabolises many medicines in the intestine (gut) or liver before they reach your bloodstream. 

This happens because of the effect of these grapefruit chemicals on an enzyme called  'CYP3A4', which is involved in metabolising many medicines. Medicines that are taken by mouth (as tablets or liquids) are affected by grapefruit in your gut. When this happens, more of the medicine may get into your body, making it work too strongly or causing unwanted side effects.

The whole grapefruit, including the juice and peel, contains furanocoumarins, the chemicals that can interact with medicines. For this reason, people who take medicines that may interact with grapefruit are usually advised not to eat grapefruit or drink grapefruit juice at all. People may also be advised 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

Some medicines – including prescription medicines, over-the-counter or complementary medicines (herbal, 'natural', vitamin and mineral supplements) – are known to interact, or potentially interact with grapefruit. .

Here are examples of some types of medicines that interact with grapefruit:

  • some statin medicines to lower cholesterol, such as simvastatin and atrovastatin
  • 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.

If you are concerned about medicine interactions

Find out whether any of your medicines interact with foods or other medicines you take by talking 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 or ask your pharmacist or doctor to print it out for you.

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 and suggest an alternative treatment if appropriate.

Call the NPS Medicines Line on 1300 MEDICINE (1300 633 424) to get information about interactions with your prescription, over-the-counter and complementary medicines (herbal, 'natural', vitamins and minerals) from a pharmacist.

Call the Adverse Medicines Events (AME) Line on 1300 134 237 if you suspect that your medicine is causing a problem and you're worried about using it. AME line provides consumers with an avenue for reporting and discussing adverse experiences with medicines.