The CEN Categorial structure is defined as a minimal set of health care domain constraints to represent a biomedical terminology in a precise healthcare domain. We present in this paper the first proposal of a categorial structure within pharmacovigilance. This study is based on the analysis and definition of 639 WHO-ART terms and the construction of an ontological model in an iterative way. Starting from a basic model based on previous experience, we added new semantic categories. changed the relations until the schema allowed to represent every term in a consistent way. We plan to investigate how this lite ontological model can help for better definition of adverse drug reactions using knowledge engineering tools such as ontology editors and reasoners.