Costart, Whoart, and MedDRA have been widely used for classification of drug safety data for the purpose of analysis and retrieval. MedDRA is, however, different from the other dictionaries in many aspects, mainly in size, granularity, and its structure. The MedDRA is characterized by its hierarchical and, multiaxial features and its regular update, which can have an impact on the safety data analysis as well as a presentation of the data. The MedDRA is globally preferred as it is continuously updated by reflecting users requests, and many pharmaceutical companies and regulatory authorities required MedDRA as a compulsory for electronic transmissions. Therefore, understanding the coding system of MedDRA is very important for clinical investigators before they use it to record and report their drug safety data. An appropriate use of coding system should enhance the applicability of drug safety database.