Naranjo ADR Causality Score Calculator
Answer 10 structured questions to score the probability that a drug caused an adverse reaction, using the Naranjo Adverse Drug Reaction Probability Scale. The running total and causality category update as you go.
Result
Causality category
Doubtful
Total Naranjo score
0
Range: -4 to +13
Questions answered
0 of 10
Scale
Naranjo
The Naranjo scale is an aid to structured causality assessment. Clinical judgement and the full case context still apply.
How the Naranjo score is interpreted
9 or more: Definite
The evidence strongly supports the drug as the cause of the reaction.
5 to 8: Probable
The drug is a likely cause, with reasonable supporting evidence.
1 to 4: Possible
The drug may have caused the reaction, but the case is not conclusive.
0 or less: Doubtful
A drug cause is unlikely on the available evidence.
What is the Naranjo ADR Probability Scale?
The Naranjo Adverse Drug Reaction Probability Scale is a widely used questionnaire that helps clinicians and pharmacovigilance teams judge how likely it is that a specific medicine caused a particular adverse event. It was published by Naranjo and colleagues in 1981 and remains a common reference in case assessment because it turns a subjective judgement into a structured, repeatable score.
The scale asks 10 questions about the case, covering prior reports of the reaction, the timing between the drug and the event, what happened on dechallenge and rechallenge, plausible alternative causes, dose response, drug levels, and objective confirmation. Each answer of Yes, No, or Do not know carries a set number of points, and the points are added into a single total. That total maps onto four categories: Definite, Probable, Possible, and Doubtful.
The Naranjo scale is an aid to structured causality assessment, not a replacement for clinical judgement. It works best when the full case context is available, including the patient history, concomitant medicines, and the timeline of the reaction. Use the score to document your reasoning and to compare cases consistently, then apply professional judgement to the final conclusion.
Frequently Asked Questions
About the Naranjo scale and this calculator
What is the Naranjo scale?+
How is the Naranjo score interpreted?+
When should I use the Naranjo scale?+
Is this Naranjo calculator free and does it store my data?+
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