Drug interactions: when medicines mix

Older adults are at especially high risk of drug-drug interactions because of the prevalence of polypharmacy

'One of the first duties of the physician is to educate the masses not to take medicine." The oft-quoted aphorism from renowned medical teacher William Osler is a warning about the potential side-effects of both prescribed and over- the- counter medication. An Institute of Medicine report estimated there were between 230,000 and 284,000 treatment induced deaths in the US annually.

The particular risk of drug interactions was highlighted recently in Dublin Coroner's Court. Coroner Myra Cullinane heard evidence suggesting a 48-year-old woman's death may have been caused by the combined use of Lemsip and paracetamol tablets. The victim was probably not aware that Lemsip contains paracetamol, leading to a verdict of misadventure. An autopsy gave the cause of death as acute liver failure with cumulative paracetamol use as a possible contributory factor.

Older adults are at especially high risk of drug–drug interactions because of the prevalence of polypharmacy. Age-related changes in the function of the body’s organs that affect the efficient clearance of drugs from the system are also a major factor. One study found that about 15 per cent of older people living in the community were at risk of a drug–drug interaction in the period 2010-2011 – a doubling in risk from five years previously. Recent research has focused on important interactions involving particular types of drugs: statins ( used to treat high cholesterol); calcium channel blockers (for the treatment of heart disease); and blood thinning agents (like warfarin and newer oral anticoagulants).

Medications that interact with statins and increase the risk of statin toxicity include the antibiotic clarithromycin and a range of antifungal agents. Drugs that cause warfarin to be less effective include a number of anti-epilepsy drugs.


But there is a particular need for the public to be aware of the interaction potential of herbal medicines with prescribed medication. Our use of over-the-counter drugs would also benefit from a sharper focus.