Medication safety - assessment of likelihood of an adverse drug reaction (including allergy), for example using of the Naranjo Scale

The Naranjo scale

The Naranjo Adverse Drug Reaction Probability Scale provides a structured, scored approach to assessing the likelihood that a specific clinical event represents a genuine adverse drug reaction to a specific agent - improving consistency and reproducibility over unstructured clinical judgement alone, particularly valuable when multiple concurrent medications make causality attribution difficult
Used across clinical practice and pharmacovigilance/adverse event reporting whenever a suspected adverse drug reaction requires causality assessment, particularly in complex, polypharmacy patients

The scale scores several weighted criteria - temporal relationship to drug administration, response to dechallenge (improvement on stopping the drug) and rechallenge (recurrence on restarting, where ethically/clinically appropriate to attempt), alternative explanations, and known reaction pattern for the drug - producing a probability category (doubtful, possible, probable, definite) rather than a binary yes/no determination, reflecting the genuine uncertainty often present in causality assessment

Applied systematically when a suspected adverse reaction is identified, working through each scale criterion methodically rather than relying on gestalt clinical impression alone, particularly valuable for reactions with an unclear or ambiguous causal relationship

A higher probability score supports drug discontinuation and avoidance of re-exposure, and informs formal adverse event reporting to relevant regulatory/pharmacovigilance bodies
Documentation of the causality assessment supports future prescribing decisions and communication with other treating clinicians about the specific drug-reaction relationship

Dechallenge/rechallenge as scale criteria (with rechallenge only pursued when clinically appropriate given the risk of reproducing a potentially serious reaction), polypharmacy causality attribution difficulty, formal pharmacovigilance reporting
Rechallenge (deliberately restarting a suspected causative drug to see if the reaction recurs) is a scale criterion that should only be pursued when clinically appropriate and safe - for a severe or potentially life-threatening prior reaction, the risk of confirming causality through rechallenge is generally not justified, and causality assessment should rely on the other available criteria instead

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