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The dangers of mixing medicines

How medication mix-ups can cost money and lives.

pills in hand
Last updated: 21 July 2014


Checked for accuracy by our qualified fact-checkers and verifiers. Find out more about fact-checking at CHOICE.

Mixing medicines can be a recipe for disaster. It can be harmful and dangerous to take different types of medicines together, but many people are in the dark about how medicines interact with other pharmaceutical treatments, and which combinations should be avoided.

Adverse drug events account for more than 400,000 GP visits and 190,000 hospital admissions annually. These figures are conservative and don't necessarily include hospitalisations for injuries, such as car accidents or falls, that may have been a result of medication issues. 

While it's hard to put a total dollar cost to the health system of medication-related problems, in 2008 hospital admissions alone were estimated at $660 million. Yet about half of these hospitalisations were likely avoidable.

Who is at risk?

The most commonly implicated drugs are cardiovascular medicines, anticoagulants, anti-inflammatories and chemotherapy medicines. People most likely to be affected are those who:

  • take five or more medications together
  • use 12 or more doses per day
  • have had significant recent changes to their treatment regimen
  • require ongoing monitoring of medication effects
  • have language difficulties, problems with dexterity, poor eyesight or dementia
  • attend several different doctors.

Many elderly people are at risk of adverse events from medicines, which can result in hospital admissions. Not only are they more likely to suffer adverse drug events, but their recovery is likely to be poorer.

Why so many drugs?

People these days are relying more heavily on medications than ever before, with the average number of daily drug doses per person more than doubling in the past 20 years. There are several reasons for this:

  • The trend to treat single conditions with several drugs.
  • Risk factors for disease are increasingly managed with medication.
  • Lifestyle-related conditions, which are on the increase (such as heart disease, high cholesterol, high blood pressure and diabetes), are more conveniently treated with medicines than with a better diet or more exercise.
  • We're living longer and acquiring more health conditions along the way that need treatment.
  • Prescription medicines, while helpful in treating or preventing relevant health conditions, often have side effects, and sometimes the side-effect costs outweigh the benefits of the drug. A classic example is benzodiazepines – prescribed to help people sleep, they also increase the risk of falls and broken bones in elderly people.


Polypharmacy is the term for people taking many medications – with an accepted clinical definition of "many" being five or more. It's estimated that up to 40% of Australians over 65 are on five or more medications, sometimes with dire consequences. If all drugs have a small chance of side effects, taking different medications combined increases the overall risk and introduces the additional danger of interactions between medicines.

One example is warfarin, a commonly prescribed anticoagulant that interacts with some anti-fungal pills and creams, anti-arrhythmic drugs, thyroid drugs and diuretics, among others, to increase the risk of bleeding. Over-the-counter medicines such as aspirin and herbal medicines such as St John's wort can also interact with prescription medicines. 

On the other hand, taking a few different medicines for some conditions can help reduce the dose of each, and therefore reduces the risk of side effects. And when treating infectious diseases, such as tuberculosis and HIV, multiple drugs can reduce the development of drug-resistant strains. However, drug combinations in these sorts of situations have usually been adequately tested.


When prescribed appropriately, medicine is usually helpful for most people – but that's not always the case. When in doubt about the potential effects of a particular medicine on a patient, a doctor often relies on information from clinical trials, journal articles and drug company advice. They may also be influenced by marketing and promotions, opinion leaders and treatment guidelines that recommend prescribing for specific diseases, without taking individual circumstances into account.

There is plenty of evidence to suggest pharmaceutical representatives and other forms of promotion have a heavy influence on prescribing habits. The consequence is increased prescribing of expensive new drugs (whose benefits and side effects are often uncertain) and decreased prescribing of older, more cost-effective generic drugs.

Beers Criteria

In 1991, the Beers Criteria – a list of medicines considered inappropriate for elderly people – was developed by geriatrician Dr Mark H. Beers. It has been updated and reviewed since then, most recently in 2012. The risks of medicines on this list, which include sedatives, muscle relaxants, antihistamines and antidepressants, are greater than any potential benefits, yet a study in 2005 found 21% of Australian veterans aged 70 or older were prescribed at least one of the medicines on the list in the previous three months.

Part of the problem is that in the drug trial phase, they're likely to be tested on people with a single condition rather than those with multiple conditions taking a variety of medicines. Middle-aged men are often chosen due to otherwise reasonable health and no confounding hormonal effects. Once the drugs are out in the real world, however, unknown problems may surface. The National Prescribing Service (NPS) recommends trying a dose half that of a typical adult dose when prescribing for older people.

Unnecessary medicines

Sometimes doctors may feel it's not their responsibility to withdraw medications prescribed by another doctor, so patients may be prescribed something for a temporary condition that has since resolved. One of our members alerted us to another problem resulting from lack of clear communication between doctor and patient: that of when to stop taking medications with repeat prescriptions. This has also led to patients being prescribed a new brand of a particular medicine — or given a generic instead of a brand – but taking both the old and the new at the same time.

A review of 31 studies looking at withdrawing certain medications from older patients found that in many cases, anti-hypertensives, benzodiazepines and antipsychotics could be successfully reduced or eliminated with no adverse effects, and sometimes lead to improvement in quality of life. In another study, GPs were trained to review medications of elderly people, and following that training were able to successfully reduce the number of medications taken by their patients.

Taking medicines incorrectly

Lack of information, instructions that are difficult to understand and complex regimens can lead to patients not taking medicines correctly. Consumer Medicine Information (CMI) leaflets are available from pharmacists, doctors and the National Prescribing Service for all prescription medications (they're sometimes found in the medicine's packaging too), but are not always offered or asked for.

Nor do prescribers always get it right. Australian researchers found that 10% of adults with chronic illness were given the wrong medicine or dose in the previous year. Other research has found about 20% of prescriptions among the general community were of the incorrect dose.


So far, the problems discussed refer largely to over-medication, where too much, too many or the wrong kinds of medicines are taken. Another significant issue is under-medication. There are several reasons for people not taking medicines, including confusion and forgetfulness, or neglect on the part of their carer. Medical professionals may also have prescribed too low a dose, or have missed a condition altogether. Patients who suffer side effects may also stop taking medicines, as, conversely, do patients who feel better, before they're actually fully recovered.

It's estimated about one in five adults with chronic illness skips a dose or doesn't fill a prescription due to cost. Many older people, and others who require lots of medicine, have a concessionary status that entitles them to low-cost medicines. They pay a nominal amount for prescriptions, called a co-payment, and when they (or their family) have spent a certain amount — up to the safety net threshold – in a one-year period, prescriptions are free for the rest of that year.

Even a small increase in the co-payment amount can reduce medicine taking. In 2005, an increase from $3.70 to $4.60 had a noticeable effect on the number of prescriptions being filled, with low-income members of the community choosing to forgo certain medicines. Subsequent research on this found that the greatest drop was for medicines prescribed for chronic but asymptomatic conditions, such as:

  • anti-platelet medicine (to prevent blood clots)
  • osteoporosis treatments
  • combination (prevention and symptom control) asthma medicines and proton-pump inhibitors (used to treat gastro-oesophageal reflux).

While some of these can be replaced by over-the-counter medicines, there's no way of knowing to what extent this occurred.

The solution

There's no single solution to the problem of adverse drug events or over- and under-medication. What's needed is a greater recognition of the problems caused by polypharmacy, and a move to reduce prescription of unnecessary medicines, as well as systemic changes.

Geriatrician Professor David Le Couteur recommends the following strategies:

  • Have a single doctor — usually your GP — manage and oversee all your medical issues and medications.
  • If any new symptom occurs, consider whether this might be due to an adverse reaction to a medication.
  • Conduct a medicine audit.
  • Discuss with your doctor (or pharmacist) whether new medications are justified and current medications are still required.

Medicine audit

It's important to regularly assess all the medicines you're taking — not just prescription products, but also over-the-counter and herbal medicines, and even dietary supplements. This can reduce the likelihood of interactions or taking unnecessary or inappropriate medicines.

The US has "brown bag days", where elderly people take all their medicines to their doctor to check for necessity and inter-drug interactions, as well as expired use-by dates. This simple but effective approach could be useful here.

People whose medication regime puts them at risk — including polypharmacy, age, recent hospitalisation, recent changes to medication and/or having multiple doctors – are entitled to a Medicare-funded Home Medicines Review. In cooperation with the patient's GP, a suitably qualified pharmacist visits the patient at home, reviews their medication regimen and provides the GP with a report. The GP and patient then agree on a medication management plan. Talk to your doctor or pharmacist about this service.

Be fully informed

Take medicines only as directed. If your doctor hasn't explained how they should be taken, or you've forgotten, ask the pharmacist. For complex medication regimes, ask the doctor or pharmacist to put it in writing.

Read the prescription label when you get the medicine, and confirm your understanding of it with the pharmacist. If it's a new medicine, make sure you're aware of any possible side effects. Ask for the CMI leaflet if it's not included with the medicine. If your new medicine makes you feel unwell, tell your doctor.

And we can't stress this enough: inform your doctor of all medicines you're taking, including over-the-counter medicines and herbal remedies and supplements. They may seem harmless because they're not prescription-only, but even "natural" supplements are still made of chemicals, and they can interact with other medicines.

Lifestyle medication

Ask your doctor about lifestyle changes that may help reduce the need for medication. The majority of the 10 most commonly prescribed PBS medicines – the ones the government subsidises – are for conditions that can be caused by poor lifestyle choices, or managed with appropriate lifestyle changes.

  • Medications to reduce high blood pressure and treat angina comprise three of the top 10 medicines prescribed. Poor diet, being overweight, smoking, excessive alcohol consumption and lack of exercise all contribute to high blood pressure and heart disease.
  • Proton pump inhibitors, taken to reduce the effects of gastro-oesophageal reflux and stomach ulcers, make up two of the top 10 medicines prescribed. Dietary factors, cigarettes, alcohol, obesity and overeating can cause reflux, or make it worse, as can medications for blood pressure, anticholinergics and anti-inflammatories.
  • Three of the top 10 medicines prescribed are statins, costing taxpayers over $1 billion per year. They're taken to reduce cholesterol levels and the risk of heart disease. Lifestyle improvements, including a better diet and more exercise, could substantially reduce this bill.
  • Two types of medication used for managing type-2 diabetes, the latest lifestyle-related disease epidemic, feature in the top 10, with the main one, metformin, clocking up 3.4 million prescriptions per year.

These illnesses and conditions can't always be prevented or managed with lifestyle changes, but a high proportion could. People often find it more convenient to pop a pill than change their lifestyle, and doctors know that withholding medicine will simply mean ill-health and higher costs further along the line in the form of hospital admissions, disability and premature death.

More information

The National Prescribing Service is a government-funded body responsible for providing health professionals and members of the public with information and advice about medicines. It has a lot of useful information for consumers, including the printable, wallet-sized Medicines List and Consumer Medicine Information.

What CHOICE wants

  • Greater recognition of and adherence to Beers Criteria medicines when prescribing for the elderly.
  • Greater consideration of non-pharmacological therapies in conjunction with or instead of drugs.
  • A reduction in the role of pharmaceutical marketing to doctors and in hospitals, and reduced role of drug companies in creating treatment guidelines.
  • A review of the role of pharmacists in primary care. As more health professionals acquire prescribing rights, pharmacists will increasingly become gatekeepers for individuals' medication regimes. Research has shown closer collaboration between GPs and pharmacists, and improved medication reviews by pharmacists, can help identify and resolve medication-related problems.

Case study

Repeat scripts spark alarm

Andrew Ross* tells how taking his father for a medicine review revealed a major flaw in prescribing practices.

"My late father was an elderly man with his faculties intact. One day when visiting him, I came across his medication ritual. He would line up all his medications and take them one or two at a time, depending on the instructions. After one series of dosages, he would line up the next and set his alarm clock so as not to forget, and so on. He knew why he was taking each medication, and showed me his bathroom cabinet which was full of nothing but medicines."

Despite his father's objections, Andrew made an appointment for both of them to see his father's doctor.

"At this appointment I asked the doctor what medications my father should currently be taking. The doctor named four, all for managing his blood pressure and blood coagulation. I then told the doctor how many and what medications my father was taking regularly. The doctor was dumbfounded and asked Dad why. Dad replied, "I had the repeats and you never said to stop taking them."

The doctor then alerted everyone in the practice to their duty of care regarding prescribing medications, particularly repeat prescriptions. The doctor had assumed patients would stop taking one medicine when prescribed another.

"The doctor, a GP who specialises in geriatrics, has since discussed this with his peers and discovered almost none told patients when to stop taking medicines, unless there was a contraindication," says Andrew.

* Name changed

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