Synopsis
Explain how interactions change drug exposure or effect, then choose avoidance, substitution, dose adjustment or monitoring according to the clinical consequence.
- Check interactions when medicines are started, stopped, restarted or switched between formulations and routes.
- Pharmacokinetic interactions alter exposure; pharmacodynamic interactions change combined effects without requiring a concentration change.
- An interaction alert becomes useful only when its consequence, timing and management are understood.
Key red flags
New muscle pain or weakness with dark urine during simvastatin treatment, particularly after an interacting medicine, needs urgent assessment for muscle injury and kidney complications.
Reasoning priorities
Identify every drug and non-prescription factor that could participate.
Include intermittent analgesics, supplements, short antibiotic courses, recent discontinuations and changes in smoking. Establish actual start and stop dates. A prescription record alone may miss the newly purchased product that explains why previously tolerated treatment has become problematic.
Worked reasoning
A 63-year-old taking simvastatin 40 mg nightly is prescribed clarithromycin for an infection. He has not started the antibiotic and reports no muscle symptoms. The antibiotic prescriber has not seen his repeat list.
- Confirm the infection assessment, the precise antibiotic indication and the current statin exposure. There is time to prevent the interaction because the new treatment has not started; do not tell the person simply to watch for symptoms while taking both.
- Recognise potent inhibition of simvastatin metabolism and the resulting muscle-toxicity risk. The combination is contraindicated. Separating the doses between morning and evening will not resolve enzyme inhibition.
- Contact the treating clinician to choose an effective, clinically appropriate non-interacting antibiotic if possible. If clarithromycin is necessary, arrange a documented temporary suspension of simvastatin during the course rather than an unplanned dose reduction.
- Tell the person exactly which medicine to take, which to pause and when the long-term treatment will be reviewed or resumed. Check that the amended instructions reach the dispensing team and are not contradicted by the printed repeat label.
- Verify that no doses of the combination were taken and ask about muscle pain, weakness or dark urine. If exposure and symptoms emerge, arrange prompt clinical and laboratory assessment. Ensure the temporary interruption does not become permanent loss of indicated lipid treatment.