Recognition that an adverse drug reaction may contribute to the differential diagnosis

The principle

Any new or worsening symptom in a patient on regular medication should prompt consideration of an adverse drug reaction as part of the differential diagnosis, not just consideration of a new primary medical condition - this is a frequently overlooked diagnostic step given the natural tendency to search for a new disease process rather than reconsider existing medications
Relevant to essentially every clinical presentation in a patient on regular medication, given how common adverse drug reactions are as a cause of new symptoms, particularly in polypharmacy patients

Adverse drug reactions can mimic a very broad range of primary medical conditions (e.g. a drug-induced parkinsonism mimicking idiopathic Parkinson disease, drug-induced hepatitis mimicking primary liver disease) - failing to consider medication as a cause risks both an incorrect diagnosis and inappropriate additional treatment for a "new condition" that is actually a reversible drug effect (the prescribing cascade phenomenon, see )

A structured medication review (recent starts, dose changes, and the temporal relationship between medication changes and symptom onset) should be part of the routine differential diagnosis work-up for any new symptom, not reserved only for cases where no other cause is found
The Naranjo scale (see ) provides a structured framework once an adverse drug reaction is specifically being considered

Trial dechallenge (temporarily stopping the suspected causative drug, where clinically safe to do so) can be both diagnostically informative and therapeutic if the symptom resolves
Avoiding the prescribing cascade by considering medication cause before adding a new treatment for the "new" symptom

Drug-induced parkinsonism mimicking idiopathic disease, drug-induced conditions mimicking primary organ disease broadly, prescribing cascade as the consequence of missing this differential consideration, trial dechallenge as both diagnostic and therapeutic

Adverse drug reactions should be actively considered as part of the differential diagnosis for any new symptom in a patient on regular medication, not only reached for after other causes have been excluded - this proactive consideration prevents the prescribing cascade, where a drug side effect is misattributed to a new medical condition and treated with an additional medication rather than addressing the actual causative drug

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