The dangers of mixing medicines

CHOICE investigates adverse events associated with medication and what can be done to prevent them.
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02.Why there is a problem

People are using more medicines 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
  • lifestyle conditions 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 – an accepted clinical definition being five or more. It’s estimated 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 a combination 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 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 not always. 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 (see Promotion Overdo$e). 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 2003. The risks of medicines on this list, which include sedatives, muscle relaxants, antihistamines and antidepressants, are greater than any potential benefits. 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.


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