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Understanding drug interactions

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

6 min read

What are interactions, and when are they most likely?

Interactions are when another medicine, food or drink (including alcohol) changes how a medicine works, or changes its side effects in some way. These 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, which can happen if you are taking two medicines that do similar things in your body.

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 to 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. Although some interactions between 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. 

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 and supplements. Keeping a Medicines List will help you remember all the medicines you are taking. 

You can find a list of known interactions in the consumer medicine information (CMI) for your particular medicine in the leaflet section called ‘Taking other medicines’. Use our Medicine Finder to find 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, 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 food instructions with medicines

Instruction Explanation
Take on an empty stomach Some antibiotics, such as flucloxacillin (eg, Flopen, Staphylex), don't work as well if taken with food. However, 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 (eg, Nurofen) and naproxen (eg, 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 (eg, Vibramycin), and some osteoporosis medicines, such as alendronate (eg, Fosamax), 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, 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.

Painkillers - 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.

Mixing grapefruit with medicines

Chemicals in grapefruit can affect how medicines work in the body

It has been known for some time that certain chemicals in grapefruit (known as furanocoumarins) can interfere with the way your body metabolises or processes many medicines in the intestine (gut) or liver before they reach your bloodstream. When this happens, more of a medicine may get into your body, making it work too strongly or causing unwanted side effects.

This happens because of their effect on 'CYP3A4' — an enzyme involved in metabolising many medicines. Medicines need to be taken orally (e.g. in the form of a tablet or liquid) to be affected by grapefruit because grapefruit mainly affects this enzyme in the gut.

Some other citrus fruits might have a similar effect

Bitter oranges (e.g. Seville oranges) and limes may also cause interactions with medicines. Other citrus fruits such as sweet oranges and lemons don't have this same effect.

Many medicines are already known to interact with grapefruit

Several prescription, over-the-counter or complementary medicines (herbal, 'natural', vitamin and mineral) are known to interact, or potentially interact, with grapefruit. Common examples include some types of medicines for heart conditions, high blood pressure, high cholesterol, infections, epilepsy, depression, anxiety and sleep problems.

Side effects that are caused by these interactions

The exact side effects from interactions vary and depend on the medicine, but some are serious.

Serious side effects have occurred when certain medicines are taken with grapefruit, including:

  • complete heart block (resulting in a very slow heart beat)
  • torsade de pointes (rapid heartbeats that can lead to sudden death)
  • rhabdomyolysis (severe damage to skeletal muscle that can lead to kidney damage)
  • nephrotoxicity (kidney damage)
  • myelotoxicity (damage to bone marrow)
  • respiratory depression (reduced or slowed breathing).

Consuming any part or form of grapefruit can cause an interaction

The whole fruit including the juice and peel contain 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.

Interactions can result from just one serve of grapefruit

Even one glass of juice or one grapefruit can have an effect on a medicine, and an interaction can occur even when the grapefruit or juice is eaten or drunk at a different time. The severity of the interaction may also depend on how often you consume grapefruit or grapefruit juice.

How to avoid harmful interactions

Find out whether any of your medicines interact with grapefruit juice 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 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 and you want grapefruit 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.

6 min read

Date published: 18 January 2017
Reasonable care is taken to provide accurate information at the time of creation. This information is not intended as a substitute for medical advice and should not be exclusively relied on to manage or diagnose a medical condition. NPS MedicineWise disclaims all liability (including for negligence) for any loss, damage or injury resulting from reliance on or use of this information. Read our full disclaimer. This website uses cookies. Read our privacy policy.