Predicting interactions
Another important risk factor to consider is a medicine's therapeutic range. Interactions that involve medicines with a narrow therapeutic range (e.g. ciclosporin, lithium, phenytoin, warfarin, digoxin, theophylline) are more likely to be clinically significant. So starting a new medicine in patients taking these 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.
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 SmPC 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 SmPC may be valuable here.
Quick clinical 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.
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, SmPC, and Stockley's Drug Interactions all state that fluconazole can increase carbamazepine levels, potentially causing toxicity, and that patients should be monitored accordingly.