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Understanding drug interactions: reducing your risk

Drug interaction can occur when different medicines are taken together, or when a medicine is taken with other agents.

Close up assortment of medical tablets
If a drug interaction occurs, it may mean that one of the drugs doesn't work as well as intended or doesn't work at all

Although some interactions may be harmful, some can be beneficial too. For example, when taken together, aspirin and warfarin increase the tendency to bleed. However, if you are prescribed both, as long as the doses and the effects of treatment are carefully monitored, the combination is quite safe - and can even be beneficial.

Who's at risk?

it is widely acknowledged that some people are more at risk from interactions than others:

  • People with poor health generally are also at higher risk because their body's ability to eliminate drugs may be impaired, resulting in an increased risk of drug interaction.
  • Older people have a higher risk, partly because their capacity to absorb medication and kidney function may change with age, but also because they tend to take more medicines, increasing the risk.

How drugs interact

Sometimes a drug may combine with another, or with a food or a vitamin, in the intestine to form a compound that is not so readily absorbed, reducing the benefits of drug, food and vitamins. An example of this is when tetracycline antibiotics are taken with antacids or iron.

However, one drug may speed up the absorption of another, causing the drug to work more quickly. This may produce beneficial results - but it can also be a problem if more of the drug is absorbed.

Many drugs are inactivated in the body by enzymes in the liver. Some drugs increase the production of these enzymes while others may inhibit or reduce it. Such drugs will therefore affect the rate at which other drugs are deactivated.

In a similar way, one drug may affect the kidney's ability to excrete another, thereby raising or lowering the level of the second drug in the blood. If the level is raised too high, again there is a risk of adverse effects. If the blood level drops too low, the drug may not work properly.

Why interactions matter

If a drug interaction occurs, it may mean that one of the drugs doesn't work as well as intended or doesn't work at all.

Alternatively, it might result in an adverse effect. In such cases, you might decide to stop taking the medicine, and if you need it for a particular disease or condition, stopping is not a good idea.

Other medicines and supplements can have effects on blood tests, while some can increase the risk of nutritional deficiencies.

Reducing the risk

  • Always remind your doctor or pharmacist of any prescription medicines you are taking and your medical conditions.
  • Always tell your doctor or pharmacist about any over-the-counter medicines or vitamins you are taking, such as painkillers, cough and cold remedies, and herbal and homoeopathic remedies.
  • If you are on regular medication, find out which types of over-the-counter medicines, supplements or herbal remedies can interact with it.
  • Do not take any dietary supplements or herbal remedies without first checking if they are safe for you.
  • Always take medicines as instructed on the label.


The opinions expressed are those of the author and are not held by Saga unless specifically stated. The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.