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The Naranjo Causality Assessment, also known as the Naranjo Algorithm, is a systematic tool used in pharmacovigilance to determine the likelihood of whether an adverse drug reaction (ADR) is actually due to the drug rather than the result of other factors. This method was developed by Dr. Alberto Naranjo and colleagues in 1981 and has since become a fundamental part of post-marketing surveillance of drugs.

The Naranjo Causality Assessment provides a structured approach to evaluating potential ADRs. This is crucial because accurately identifying drug-related adverse effects can significantly impact patient safety and treatment outcomes. By applying this algorithm, healthcare providers can make informed decisions about whether a drug should be discontinued, its dosage adjusted, or if an alternative medication should be considered.

How does the Naranjo Causality Assessment work?

The Naranjo Scale consists of a series of questions designed to gather information about the relationship between the drug and the adverse event. These questions focus on factors such as the temporal relationship between drug administration and the onset of the reaction, previous patient exposure to the drug, alternative explanations for the reaction, and whether the reaction improved when the drug was discontinued.

Each question in the Naranjo Scale has multiple-choice answers with corresponding scores. The scores are typically 0, 1, or 2, reflecting the degree of certainty or uncertainty regarding the drug's causality. For example, questions about the time course of the reaction and its improvement upon drug withdrawal may have options like "definite," "probable," "possible," or "doubtful," each with its associated score.

After completing the questionnaire, the scores from each question are totalled to obtain a cumulative score. The total score ranges from -4 to +13, with interpretations based on predefined cutoffs. Higher scores indicate a higher probability that the drug caused the adverse event, while lower scores suggest a lower likelihood of causality

The Naranjo Scale provides guidelines for interpreting the total score:

< 0 : Indicates that the adverse event is unlikely to be caused by the drug.

1-4 : Suggests a possible relationship betwe 0en the drug and adverse event.

5-8 : Indicates a probable causal relationship, with increasing confidence as the sore approaches 8.

> 8 : Considered as a definitive or highly probable link between the drug and the adverse event.

Clinical judgment and utility in pharmacovigilance

While the Naranjo Scale provides a structured approach, it also emphasizes the importance of clinical judgment in interpreting the results. Factors such as the patient's medical history, concurrent medications, underlying conditions, and the nature of the adverse event must be considered alongside the Naranjo score to arrive at a comprehensive assessment of causality.

The Naranjo Scale is valuable in pharmacovigilance services for identifying potential adverse drug reactions, assessing their likelihood of being drug-related, and guiding further investigation or management strategies. It aids healthcare professionals in making informed decisions about drug safety and risk mitigation.


The Naranjo causality assessment offers a structured and systematic approach to evaluating the probability of a drug causing an adverse event, enhancing the reliability and consistency of pharmacovigilance practices.

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