How it's calculated
Each of the 10 questions is answered Yes / No / Don't know, and each answer carries its own point value (some questions score No higher than Yes, e.g. "are there alternative causes?"). Points are summed to a total score, which maps to one of four causality categories.
Score interpretation
| Score | Causality category |
|---|---|
| ≥ 9 | Definite |
| 5–8 | Probable |
| 1–4 | Possible |
| ≤ 0 | Doubtful |
Clinical use
- Used in pharmacovigilance and clinical practice to structure the causality assessment of a suspected adverse drug reaction (ADR), rather than relying on subjective judgement alone.
- A higher category (Probable/Definite) supports a stronger case for documenting the reaction as a true drug allergy/intolerance and avoiding re-exposure.
- Does not replace formal pharmacovigilance reporting — a Naranjo assessment should still be reported through the appropriate national/institutional ADR reporting system regardless of the calculated category.
- Best applied with as much clinical detail as possible — "Don't know" answers push the total toward a lower, less certain category, which is an intentional feature of the scale, not a flaw.
Frequently asked questions
What is the Naranjo scale used for?
It estimates the probability that a specific drug caused an observed adverse event, using a structured 10-question algorithm, classifying the reaction as Definite, Probable, Possible, or Doubtful.
What score counts as a "definite" adverse drug reaction?
A total score of 9 or more. Probable is 5-8, Possible is 1-4, and Doubtful is 0 or less.
Why does "No" sometimes score higher than "Yes"?
For questions like "are there alternative causes that could explain the reaction?", answering No (i.e. no good alternative explanation exists) increases the likelihood the suspected drug is responsible, so No scores +2 while Yes scores -1 for that specific question.
Does a low Naranjo score mean no adverse reaction occurred?
No — it means the available evidence doesn't strongly support that specific drug as the cause, often because rechallenge/dechallenge information or objective confirmation is unavailable. The reaction itself may still be real; the score is about causality attribution to the suspected drug specifically.
References
- Naranjo CA, et al. A method for estimating the probability of adverse drug reactions. Clin Pharmacol Ther. 1981;30(2):239-245.