MedDRA (Medical Dictionary for Regulatory Activities)

MedDRA (the Medical Dictionary for Regulatory Activities) is the standardized, hierarchical medical terminology used to code signs, symptoms, diseases, diagnoses, and other clinical findings — most consequentially, adverse events and adverse reactions — for entry into a clinical trial database and for transmission to regulators. It was developed under the International Council for Harmonisation (ICH) and is maintained by the Maintenance and Support Services Organization (MSSO) on ICH’s behalf. An event only becomes fit for aggregation, comparison across trials, or electronic submission once it has been mapped to a MedDRA term; a study’s own free-text description of what happened to a subject (“felt queasy after dosing”) is not itself submission-ready data.

What MedDRA is for

Clinical trial staff and investigators record adverse events in whatever language a subject or clinician uses. Regulators, sponsors, and safety reviewers need that language normalized into a controlled vocabulary so that events can be counted, grouped, and compared consistently across sites, trials, and even different sponsors’ submissions. MedDRA is that controlled vocabulary. A MedDRA-coded term is required for electronic individual case safety report (ICSR) submission to regulators across the US, EU, Japan, and other ICH-participating jurisdictions, and MedDRA coding underlies the adverse-event tables in a trial’s clinical study report, its Development Safety Update Reports, and its post-marketing pharmacovigilance case processing. See CASRAI’s Pharmacovigilance in Clinical Research guide for the reporting timelines and obligations (AE, SAE, SUSAR) that MedDRA-coded terms feed into — this entry focuses on the terminology and coding structure itself, which that guide does not cover in depth.

The five-level hierarchy

MedDRA organizes terms into five hierarchical levels, running from broadest to most granular:

Level	Name	Role
1 (highest)	System Organ Class (SOC)	The broadest grouping, by anatomical/physiological system, etiology, or purpose (e.g., “Gastrointestinal disorders”).
2	High Level Group Term (HLGT)	An intermediate grouping beneath a SOC (e.g., “Gastrointestinal signs and symptoms”).
3	High Level Term (HLT)	A more specific grouping beneath an HLGT (e.g., “Nausea and vomiting symptoms”).
4	Preferred Term (PT)	A single, distinct medical concept used as the standard reporting term (e.g., “Nausea”).
5 (lowest)	Lowest Level Term (LLT)	The most granular level, capturing how a term is actually expressed in source data, including synonyms and colloquialisms (e.g., “Feeling queasy”), each mapped to exactly one Preferred Term.

A coder maps the reported event to the most appropriate LLT; MedDRA’s structure then automatically carries that mapping up through its linked PT, HLT, HLGT, and SOC, which is what allows a safety reviewer to run analyses at whatever level of granularity a question requires — a specific LLT for case-level detail, or a SOC for a high-level signal-detection view across an entire safety database. A Preferred Term is not necessarily linked to only one SOC: MedDRA’s “multiaxial” design allows a PT to appear under more than one SOC where clinically appropriate, with a single “Primary SOC” designated for default reporting and analysis so that counts aren’t inflated by double-counting the same event under multiple system classes.

What MedDRA is not

MedDRA codes what happened, not how severe it was and not what caused it. Severity grading is a separate function, most commonly performed with the NCI’s Common Terminology Criteria for Adverse Events (CTCAE) in oncology trials — a coder might assign the MedDRA PT “Nausea” to an event and separately grade it CTCAE Grade 2, and the two classifications serve different purposes and should never be conflated. MedDRA also does not code the product involved: drug and medication names are coded separately, most commonly with WHODrug, the medicinal-product dictionary maintained by the Uppsala Monitoring Centre. A single ICSR typically carries both a MedDRA term for the event and a WHODrug (or equivalent) term for the suspect product.

Governance and versioning

MedDRA is developed and maintained under ICH governance, with day-to-day maintenance, updates, and user support delegated to the MSSO; a separate Japanese Maintenance Organization (JMO) supports the Japanese-language version. MedDRA is updated on a regular release cycle to add new terms and refine existing ones, which is why a study’s coding conventions typically specify which MedDRA version was used — case counts and even hierarchy placement can shift slightly between versions, so version control matters for anyone comparing safety data coded at different points in a product’s development.

Frequently asked questions

Is MedDRA the same as ICD-10?

No. ICD-10 is a diagnostic classification system built primarily for billing, epidemiology, and mortality statistics, with a much coarser code structure. MedDRA was purpose-built for coding adverse events and other clinical findings in regulated drug and device development, with far greater granularity at its lower levels (the LLT layer in particular) to capture how events are actually described in source data.

Who assigns the MedDRA code to an adverse event?

Trained medical coders (often within a sponsor’s safety or clinical data management function, or at a contract research organization performing that role) map each reported event to the appropriate Lowest Level Term, following documented coding conventions to keep mapping decisions consistent across coders and over the life of a trial or product.

Does every clinical trial need to use MedDRA?

Any trial that will generate a regulatory submission in an ICH region needs MedDRA-coded safety data, since electronic ICSR submission requires it. Purely exploratory, non-regulated research without a submission pathway may use other terminologies, but MedDRA is the de facto standard wherever a regulatory safety-reporting obligation exists.

Related CASRAI resources

See the Pharmacovigilance in Clinical Research guide for the AE/SAE/SUSAR reporting framework that MedDRA-coded terms flow into, and the dictionary entries for Good Pharmacovigilance Practices (GVP), ICH, and CTCAE. For the wider picture, see the Clinical Research Administration cluster hub.