Medicine interactions explained Medicine interactions explained

The work of a medicine is done by its active ingredient, which is the chemical in the medicine that produces the intended effect.

But, what happens if you are taking more than one medicine, and so have more than one active ingredient in your body?

What happens?

Most of the time, each of the active ingredients works separately as intended, and each produces the desired changes in your body. 

However, sometimes, two active ingredients interfere with each other, which results in one of the medicines working more strongly or less strongly than intended.

Most of the time, the effect of the interaction is too small to be noticeable. Occasionally, the change is big enough for the interaction to have a significant effect on the body.

For example, an interaction with a blood pressure medicine could cause it to work too strongly, lowering your blood pressure so much that you feel lightheaded or faint when you stand up suddenly.

Conversely, an interaction could cause your blood pressure medicine to work too weakly, so the medicine does not lower your blood pressure enough.

An interaction may also make you more likely to experience other side effects from the blood pressure medicine or increase the severity of those side effects.

Experiencing an interaction does not necessarily mean that you have to stop the medicine that is causing the interaction. In some cases, interactions may be dealt with by simply adjusting the dose of one or both medicines.

What medicines can interact?

Interactions are not just confined to prescription medicines. All types of medicines can interact with each other, and with some foods and drinks.

Tell your doctors and pharmacists about all your medicines

so they can take potential interactions into account.

Who is most at risk?

People who take multiple medicines are more likely to experience interactions, because they have more medicines in their bodies.

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.

Interactions are most likely to cause problems when you:

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

Therefore, you should watch out for unexpected symptoms in the first few days after your medicines change in any way.

Avoiding interactions

Tell your health professionals about all your medicines

Most interactions between medicines are well known. You can minimise your chance of having an interaction by telling your health professionals about all the medicines you are taking each time your medicines are changed in some way. That way, any potential interactions can be considered when choosing or recommending a medicine for you.

All your medicines means:

• any medicine prescribed by a doctor or specialist
• any medicine bought at a pharmacy, health food store or supermarket
• any medicine prescribed by an alternative health practitioner
• any traditional medicines you are taking, including creams and herbal medicines.
  

Telling your health professionals about all your medicines is easier if you have a list of what you take. If you don’t have a medicines list, take the time to make one, or ask your doctor or pharmacist to help you. You might like to write them on the NPS Medicines List, which can be ordered from the NPS website.

Keep it in your wallet or handbag, so you have it on hand whenever you need it. Remember to update the list if your medicines change.

Read the CMI

The medicine’s Consumer Medicine Information (CMI) leaflet has information about possible interactions. To obtain the CMI leaflet, ask your pharmacist for a copy, or read or download it from the NPS website.

Read the label

Non-prescription medicines bought from pharmacies, health food shops and supermarkets have warnings about their use on the label. Check the label for medicines that might interact with them before buying. If you’re unsure whether the information applies to you, ask a pharmacist for advice.

Also, before taking any medicines you have not used recently, check the label.

Particular attention needs to be given to cold and flu tablets and decongestants. Many of these products contain active ingredients that can interact with some prescription medicines.

Have your medicines reviewed

If you take several medicines, it may be worthwhile asking your GP for a Home Medicines Review. A Home Medicines Review is a check of all your medicines by a specially trained pharmacist, including a check for possible interactions.

If you suspect an interaction

If you think you may be having an interaction, talk to your doctor or pharmacist. They can tell you whether the symptoms might indicate an interaction, and advise you what to do next.

Susan’s story

Susan is 73 and has had Type 2 diabetes for 15 years. She takes metformin* and gliclazide** for her diabetes. Her blood sugar is well controlled. Recently, Susan developed pain on passing urine and went to the doctor. She was seen by a doctor she had not seen before who diagnosed a urinary tract infection, and prescribed an antibiotic. Two days later, Susan’s daughter Karen visited her, and found her drowsy, confused, pale and sweaty. Karen tested her mother’s blood glucose level, and found that it was very low. When Karen took her mother to her regular doctor, they were told that Susan was experiencing an interaction between gliclazide and the antibiotic #. The interaction had caused the gliclazide to work more strongly than usual, which had lowered Susan’s blood sugar level too much and caused the drowsiness and other symptoms. Her regular doctor stopped the antibiotic, and gave her a different one. He told Susan that, to reduce her chance of having an interaction, she should always tell her doctor and pharmacist about all the medicines she is taking whenever she starts a new one. *     Metformin brand names include Diabex, Diaformin, Glucophage, Glucohexal. **    Gliclazide brand names include Diamicron, Nidem, Gliclazide, Glyade. #     This interaction occurs only between gliclazide and the particular type of antibiotic Susan was prescribed.