Naranjo Adverse Drug Reaction Probability Scale
The Naranjo ADR Probability Scale was developed to help standardize assessment of causality for all adverse drug reactions. The scale was also designed for use in controlled trials and registration studies of new medications, rather than in routine clinical practice. 
Research authors: Naranjo CA, Busto U, Sellers EM, Sandor P, Ruiz I, Roberts EA, Janecek E, Domecq C, Greenblatt DJ
Details Formula Study characteristics Files & References
Model author
Model ID
Revision date
MeSH terms
  • Adverse Drug Reaction
  • Pharmacovigilance
  • Clinical Pharmacology
  • Model type
    Linear model (Calculation)
    No Formula defined yet
    Condition Formula

    Additional information

    Clinical Pharmacology and Therapeutics (1981) 30, 239–245; doi:10.1038/clpt.1981.154

    Study Population

    Total population size: 0

    Additional characteristics

    No additional characteristics defined
    Naranjo Adverse Drug Reaction Probability Scale
    Refer to Intended Use for instructions before use
    Evidencio B.V., Irenesingel 19, 7481 GJ, Haaksbergen, the Netherlands

    Related files

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    Supporting Publications

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    Outcome stratification

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    Conditional information

    Result interpretation


    • Doubtful ADR (<2): The reaction was likely related to factors other than a drug).
    • Possible ADR (2 to 4): The reaction followed a temporal sequence after a drug, possibly followed a recognized pattern to the suspected drug and could be explained by characteristics of the patient’s disease.
    • Probable ADR (5 to 8): The reaction followed a reasonable temporal sequence after a drug, followed a recognized response to the suspected drug, was confirmed by withdrawal but not by exposure to the drug, and could not be reasonably explained by the known characteristics of the patient’s clinical state.
    • Definite ADR (≥9): The reaction followed a reasonable temporal sequence after a drug or in which a toxic drug level had been established in body fluids or tissues, followed a recognized response to the suspected drug and was confirmed by improvement on withdrawing the drug and reappeared on reexposure.

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    Calculations alone should never dictate patient care, and are no substitute for professional judgement. See our full disclaimer.

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