Predicting interactions

Last updated: Thursday, November 23, 2017

An important part of the pharmacist's role is to anticipate when interactions are likely to occur. For example, patients taking lots of drugs (polypharmacy) are more likely to experience an interaction.

Another important risk factor to consider is medicines with a narrow therapeutic range (e.g. ciclosporin, lithium, phenytoin, warfarin, digoxin, theophylline). In patients who take these medicines, an interaction which affects the drug is more likely to be clinically significant. So starting a new drug in these patients always requires a check for interactions.

There are a number of ‘alarm bell’ drugs that are particularly liable to interact with many other medicines (e.g. erythromycin, ritonavir, carbamazepine, fluoxetine, rifampicin, clozapine). Try to have a mental list of drugs like these to guide you in your practice.

Quick ward question 
A doctor asks if it's OK to prescribe fluconazole to patient on carbamazepine. Where would you look for info?    Consider, then click for answer.
BNF, SPC, and Stockley's Drug Interactions all say fluconazole can increase carbamazepine levels, potentially causing toxicity, and that patients should be monitored accordingly.

Nonetheless, it is not always easy to discover whether an interaction will or will not occur in clinical practice. If you can’t find any published information, you may be able to predict the likelihood of an interaction, using these points as a guide:

  • How are the two medicines cleared from the body? Are they metabolised by the same enzyme? Is one of the drugs known to inhibit or induce it? The SPC will often provide this kind of information. 
  • Is there any information on how pairs of related drugs behave when given together? Stockley’s Drug Interactions may assist you with this. 
  • Do the two drugs have any frequent side effects in common? These effects may be additive when the two drugs are given together. Again, the SPC may be valuable here.