The strange world of drug interactions

Dr Chris Freeman and Dr Geraldine Moses from the University of Queensland remind pharmacists that many drug interactions are not yet known – some of which can be quite unexpected

When a patient approaches you with an unusual symptom, how often do you consider drug interaction as the source?

There is still a lot unknown about medications and in particular how multiple medicines interact with one another, warns Dr Freeman, a clinical lecturer at UQ’s School of Pharmacy.

“We can often predict interactions based on each medication’s pharmacology; however, there may be some interactions which occur that cannot be explained based on the known pharmacology.”

For example, have you heard about the possible interaction between the SSRI paroxetine and pravastatin?

According to papers published over the past few years, combining the two drugs could have a range of results, from raised blood glucose levels to renal impairment.

One retrospective study measuring changes in blood glucose levels in patients with diabetes and without found that pravastatin and paroxetine, when administered together, had a synergistic effect on blood glucose.

“You wouldn’t guess this is something these drugs would do – something so unexpected,” says Dr Moses, who is a senior clinical and drug information pharmacist and Adjunct Associate Professor with UQ’s School of Pharmacy.

Regarding this particular study, Dr Moses and Dr Freeman point out that random blood glucose levels are highly variable and often reflect what food has been recently consumed, and other studies found no interaction between the two drugs.

However, Dr Moses says further studies that covered both humans and rats found taking paroxetine and pravastatin together affect not just blood glucose levels but the kidneys – “suggesting that they’re actually going into renal impairment,” she told AJP.

“We need to ask – what’s the pathology behind that? And what part of their body is going awry?

“These additional studies suggest the interaction could be a much bigger issue than just elevated glucose levels,” says Dr Moses.

Approaching drug interaction

The potential interaction between paroxetine and pravastatin is not generally listed in medicine interaction databases.

It reveals a knowledge gap for pharmacists that they need to fill with self-education, says Dr Freeman.

“This article highlights that if a patient presents with an unusual reaction potentially caused by two or more medications that pharmacists should search the primary literature to see if case reports or emergent evidence has been published on this topic; rather than just relying on medication interaction checkers/databases,” he says.

“Pharmacists should remain vigilant when a patient presents with an adverse or an unusual effect which is not commonly attributed to the medicines they are on.

“As medicine experts, pharmacists have a responsibility to stay up to date with new information regarding medications and their potential for interactions.”

Dr Moses agrees, adding that these studies reveal why pharmacists are so important in a multidisciplinary team.

“Someone from a health background would assume the person was getting diabetes, whereas a pharmacist – someone who specialises in drugs – would see it from a different perspective.

“I don’t think we should expect pharmacists would know about this drug interaction, but that’s why it’s important for them to keep updated with the literature,” she says.

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1 Comment

  1. Ash Blackwell

    The other aspect of poly-drug interactions is the sometimes appalling gap in knowledge regarding interactions between prescription and recreational drugs displayed by some doctors and pharmacists. This knowledge is often easily available and poorly understood. Even when patients openly disclose their drug habits, from personal experience, but I’ve raised this in public forums and am assured it’s not a unique experience.

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