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BACKGROUND: Pharmacovigilance centres (PVCs) in the World Health Organization (WHO) Programme for International Drug Monitoring have demonstrated their ability to detect preventable adverse drug reactions (ADRs) in their databases. In this field, there is no gold-standard method for detecting medication errors and evaluating ADR preventability. Therefore, we developed, from existing tools, a preventability assessment method: the 'P Method' (PM). OBJECTIVE: To present the PM and to evaluate its inter-rater reliability. METHODS: The PM includes 20 explicit criteria for assessing ADR preventability. This approach is based on identification of any potentially preventable risk factor that increases the likelihood of ADR occurrence. The outcome of the preventability assessment results in one of three possible scores: 'preventable', 'non-preventable' or 'not assessable'. The PM was tested in a multicentre study involving nine national PVCs. Two experienced reviewers at each participating PVC independently analysed the preventability of 183 ADRs, applying the PM. RESULTS: The overall agreement between all reviewers for assessment of ADR preventability was 'fair', with a kappa value of 0.27 [95 % confidence interval (CI) 0.21-0.40]. The level of agreement between reviewer pairs ranged from 'slight', with a kappa value of 0.12 (95 % CI -0.03 to 0.27), to 'substantial', with a kappa value of 0.69 (95 % CI 0.48-0.89). CONCLUSION: The analysis of the agreements and disagreements between reviewers highlighted where improvements might be made. Given that no standard assessment tool exists in the WHO Programme, the transparency of the assessment process in this method provides a substantial basis for further development and for support in signalling possible preventability.