MedDRA provides the standardized terminology that enables consistent coding of adverse events, medical history, and other clinical observations across clinical trials, post-marketing surveillance, and regulatory submissions worldwide. Developed under the auspices of ICH and maintained by the MedDRA Maintenance and Support Services Organization, this hierarchical dictionary ensures that safety data can be aggregated, compared, and analyzed across different studies, sponsors, and regulatory jurisdictions using a common vocabulary.

The MedDRA hierarchy comprises five levels of increasing specificity. System Organ Classes represent the highest level, grouping terms by etiology, manifestation site, or purpose. High Level Group Terms and High Level Terms provide intermediate groupings that facilitate data retrieval and review. Preferred Terms represent distinct medical concepts used for clinical data entry and analysis, while Lowest Level Terms provide the most specific terminology available. This hierarchical structure allows safety data to be analyzed at multiple levels of granularity, from broad system organ class comparisons to specific preferred term frequencies.

Medical coding using MedDRA occurs as part of the data management process, with trained coders assigning appropriate terms to verbatim adverse event descriptions reported by investigators. The selection of the most appropriate term requires clinical judgment and adherence to coding conventions, as inconsistent coding can obscure true safety signals or create artificial patterns. Regular version updates to MedDRA necessitate careful management to ensure that coding within a study remains consistent while also incorporating important new terms that may better represent observed events.