Medical Dictionary for Regulatory Activities (MedDRA)

MedDRA is the terminology that turns free text into countable safety data. It has five levels. A Lowest Level Term captures the verbatim expression as reported; that maps to a Preferred Term, the level at which most analysis happens; Preferred Terms group into High Level Terms and then High Level Group Terms; and everything sits under one of twenty-seven System Organ Classes. It is maintained under ICH and republished twice a year.

Everything downstream depends on it. Trial safety tables, FAERS, EudraVigilance and periodic safety reports are all MedDRA-coded, which makes them searchable and poolable. Version changes matter: when a term is added, split or reassigned between releases, counts shift for reasons unrelated to the drug, which is why safety tables state the version used. Standardised MedDRA Queries exist because single clinical phenomena fragment across many Preferred Terms; a query for anaphylactic reaction or drug-induced liver injury assembles them into one defensible search.

Coding granularity therefore decides whether a signal is visible. A gastrointestinal effect spread across nausea, vomiting, dyspepsia, abdominal discomfort and decreased appetite can leave each term below the reporting threshold while the phenomenon affects a large minority of patients. Grouping restores it. Over-grouping does the opposite, merging unrelated events under a broad heading and manufacturing an apparent excess.

The practical trap is comparing incidence figures between two products' labels as though MedDRA made them commensurable. Sponsors code the same verbatim report differently, use different dictionary versions and choose different aggregation levels. In spontaneous reporting the coded term reflects what a reporter wrote, often a patient rather than a clinician, so a Preferred Term records a description, not a diagnosis.