Understanding interactions

Sometimes one medicine can mix badly with another in your body, and this can change how strongly the medicine works or change its side effects. This is called an interaction, and it can also happen when medicines mix with certain foods or drink (including alcohol). Read what you can do to avoid interactions.

Will it mix OK with my other medicines?

What is an interaction? This is when another medicine, food or drink (including alcohol) changes how strongly a medicine works, or changes its side effects in some way. These interactions (also called drug interactions) may be serious.

Interactions can be caused in many ways, but the most common are:

  • by making your body absorb or get rid of a medicine slower or faster than usual; this can mean your usual dose is either too strong or not strong enough
  • by adding together two or more medicines that when combined will give you too strong an effect; this can happen if you are taking two medicines that do similar things in your body, as in the example below.

If you are at high risk of having a stroke you may be prescribed warfarin (Coumadin, Marevan) to thin your blood so it is less likely to clot.

Taking other medicines that also thin your blood, such as aspirin and ibuprofen, can make your blood too thin. This can mean your blood doesn’t clot when it should and that you are more prone to bleeding (e.g. from your stomach or bowels).

Many other medicines also interact with warfarin, and can make your blood either too thin or not thin enough – so it is important to make sure you tell your health professional about all the other medicines you are taking.

When are interactions most likely?

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.

Video transcript

Another important thing is to know which medication does what, and the other thing is to at least get your medications checked up annually. As an example, I have a cousin who visited me from Perth, and I was absolutely aghast when, as we had breakfast, his wife emptied out into a saucer 30 tablets. I said, ‘You have to have to get that checked’. He hadn’t had anything checked for two or three years, and he had been to different GPs.

He took my advice, and he did go to a physician, and those 30 medications were cut down to 12. So, it’s a good idea to get an annual check on your medications, especially when there are a lot. It is very important.

Interactions between different medicines

Many, but not all, medicines can interact with each other. Although some interactions between certain medicines are well known, it can be difficult to predict whether a specific person taking the medicine will be affected. The more medicines you are taking, the more likely interactions are. Also, older people, people with some chronic illnesses and young children are more likely to experience interactions, because their bodies do not handle medicines as well as other people. But they can happen to anybody who takes a combination of medicines.

Always tell your health professional about any other medicines you are using, including any complementary or alternative medicines and supplements. Keeping a Medicines List will help you remember all the medicines you are taking. For more about interactions with complementary medicines, see Sometimes complementary and prescription medicines don’t mix well.

You can find a list of interactions in the consumer medicine information (CMI) for your particular medicine in the leaflet section called ‘Taking other medicines’. Search for the CMI for your medicine.

Interactions between medicines and food

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

All medicines have their own particular instructions about when to 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. Also, check for any instructions on the medicine’s label and in its CMI leaflet in the ‘How to take it’ section.

Examples of food instructions with medicines

Example of instruction Explanation and some examples
Take on an
empty stomach
Some antibiotics, such as flucloxacillin (e.g. Flopen, Staphylex), don't work as well if taken with food.

However, please talk to your health professional if you find the medicine is causing an upset stomach when taken without food.

Take with meals
Many common arthritis medicines and painkillers, such as aspirin, ibuprofen (e.g. Nurofen) and naproxen (e.g. Naprosyn), should be taken with meals to reduce the likelihood of developing 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 (e.g. Vibramycin), and some osteoporosis medicines, such as alendronate (e.g. Fosamax), don't work as well when taken at the same time as calcium-rich foods, such as milk and yoghurt, and calcium supplements.
Don't eat certain foods at all
Grapefruit juice interacts with several common medicines, making them work too strongly or causing unwanted side effects. People taking these medicines are advised not to eat grapefruit or drink grapefruit juice at all, because even one glass of juice can have an effect, and the interaction can occur even when the grapefruit or juice is eaten or drunk at a different time. Examples of medicines that interact with grapefruit juice.

Other citrus juices like orange juice and lemon juice do not have the same effect.

Eat about the same amount of certain foods every day
Vitamin K interacts with the blood-thinning medicine warfarin (Coumadin, Marevan), and affects its ability to thin the blood. People taking warfarin should eat about the same amount of vitamin K-rich foods each day (but not stop eating them — they're good for you!), so the effect of the warfarin is the same every day too. Vitamin K-rich foods include green vegetables such as spinach, silver beet, broccoli, parsley, brussels sprouts, butter lettuce and endive.

Grapefruit juice interacts with some medicines, including:

  • amiodarone (e.g. Cordarone X), which controls certain heart rhythm problems
  • simvastatin (e.g. Lipex, Zocor) and atorvastatin (e.g. Lipitor), which are cholesterol-lowering medicines
  • carbamazepine (e.g. Tegretol), an epilepsy medicine
  • cyclosporin (e.g. Neoral, Sandimmun), a medicine used to prevent rejection after an organ transplant and sometimes to treat other conditions
  • felodipine (e.g. Felodur, Plendil), a blood pressure medicine
  • tacrolimus (Prograf), a medicine used to prevent rejection after an organ transplant.

To find out whether your particular medicine interacts with grapefruit juice, read the 'Taking other medicines' section of the consumer medicine information (CMI) leaflet. You can download the CMI for your medicine from the NPS Medicine Finder or ask your pharmacist or doctor to print it out for you.

You can also 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.

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, the amount of alcohol consumed, and also on personal factors, such as genetics, gender and other health conditions. In general, women and older people are more likely to experience such interactions, because they are more susceptible to the effects of alcohol.

It can take several hours for alcohol to be removed from the body. Therefore, interactions don't occur only when you consume medicines and alcohol at the same time. Rather, they can occur at any time that you have a significant amount of alcohol in your body.

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. The increased drowsiness and dizziness may make it harder for you to think clearly and affect your physical co-ordination. This may make you more prone to falling and impair your ability to do things like drive a car.

Cough, cold, allergy and travel sickness medicines

Many cough, cold, allergy and travel sickness medicines bought from pharmacies contain ingredients that relax or sedate the body. These ingredients may interact with alcohol to cause increased drowsiness and dizziness.


Some common painkillers, such as aspirin, celecoxib (Celebrex), ibuprofen (e.g. Nurofen) and naproxen (e.g. Naprosyn), can interact with alcohol to cause stomach upsets, stomach bleeding and ulcers.

In general, the occasional 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.

Opioid painkillers, such as morphine (e.g. MS Contin) and oxycodone (e.g. OxyContin), can cause drowsiness, and may interact with alcohol to cause increased drowsiness and greater sensitivity to alcohol.