What is the difference between MedDRA and Whodrug?

October 10, 2025 •

4 min read

Over 7,500 organizations use MedDRA, and thousands more rely on WHODrug, highlighting the widespread need for standardized medical and drug terminology. While both are essential tools in clinical research and pharmacovigilance, knowing what is the difference between MedDRA and Whodrug is crucial, as they serve entirely different, yet complementary, functions.

Quick Summary

MedDRA is a standardized medical terminology for coding adverse events and patient medical history, while WHODrug is a drug dictionary for coding medicinal products and active ingredients. They are distinct dictionaries used together for comprehensive safety analysis and regulatory reporting.

Key Points

Core Distinction: MedDRA codes medical events, such as adverse reactions and diagnoses, while WHODrug codes medicinal products, including active ingredients and trade names.
Hierarchical Structure: MedDRA employs a five-level, multi-axial hierarchy based on medical concepts, whereas WHODrug uses the Anatomical Therapeutic Chemical (ATC) classification system based on drug properties.
Purpose in Pharmacovigilance: MedDRA is used for standardizing the reporting of adverse event outcomes, and WHODrug is used for standardizing the identification of the drug causing or suspected of causing the event.
Used Together for Analysis: In practice, both dictionaries are used in tandem within clinical trial and pharmacovigilance systems to link reported medical events to the specific medicinal products involved.
Different Maintainers and Update Cycles: MedDRA is maintained by the MSSO with bi-annual updates, while WHODrug is managed by the UMC with quarterly updates.
Regulatory Compliance: Both are global standards required or recommended by regulatory authorities like the FDA and EMA for submissions related to drug safety and clinical data.

Understanding the Core Functions

In the world of drug safety and regulatory science, clear, standardized communication is paramount. Both MedDRA and WHODrug are critical dictionaries used for coding data, but they address different facets of the clinical and post-market surveillance process. The primary distinction lies in what each dictionary codes: MedDRA focuses on the medical event, whereas WHODrug focuses on the medicinal product.

What is MedDRA?

MedDRA, the Medical Dictionary for Regulatory Activities, is a clinically validated, multilingual medical terminology that is an international standard for regulatory communication. It was developed under the auspices of the International Council for Harmonisation (ICH) to standardize the coding, retrieval, and analysis of safety data related to the use of medical products.

MedDRA's Hierarchical Structure

MedDRA is organized into a five-level hierarchy, allowing for data to be viewed with varying degrees of specificity:

Lowest Level Term (LLT): The most specific level, which includes synonyms, abbreviations, and lexical variants of medical concepts.
Preferred Term (PT): A single medical concept that links to one or more LLTs. This is the standard for reporting adverse events.
High Level Term (HLT): Groups clinically related PTs.
High Level Group Term (HLGT): A superordinate grouping of HLTs.
System Organ Class (SOC): The highest, most general level of the hierarchy, grouping terms by anatomy, pathology, or function. A key feature is its multi-axial structure, meaning a single term can appear in more than one SOC, reflecting clinical reality.

Key Use Cases for MedDRA

Adverse Event Reporting: MedDRA is used to code adverse events reported in clinical trials and post-marketing surveillance.
Signal Detection: Standardized MedDRA Queries (SMQs), which are pre-defined groups of terms, help in identifying safety signals.
Regulatory Submissions: It is the required terminology for submitting safety data to regulatory authorities like the FDA and EMA.
Medical History and Indications: Used to code patient medical and social history, as well as therapeutic indications.

What is WHODrug?

WHODrug, or the WHO Drug Dictionary, is a comprehensive drug reference dictionary that standardizes medicinal product information. Maintained by the Uppsala Monitoring Centre (UMC), a WHO Collaborating Centre, it was created in the wake of the thalidomide tragedy to enable systematic drug monitoring.

WHODrug's Data Structure

WHODrug is built around the Anatomical Therapeutic Chemical (ATC) classification system. Its structure includes:

ATC Codes: A five-level hierarchical code that classifies active ingredients based on their anatomical site of action, therapeutic use, pharmacological properties, and chemical structure.
Trade Names and Ingredients: Links specific medicinal trade names to their active ingredients, pharmaceutical form, strength, and country of sale.
Standardised Drug Groupings (SDGs): Analytical groupings that allow users to identify drugs with similar properties beyond the ATC system.

Key Use Cases for WHODrug

Concomitant Medication Coding: It is widely used to code concomitant medications (medications taken alongside the study drug) in clinical trials.
Post-Marketing Surveillance: Enables the identification and aggregation of medicinal products in safety databases like VigiBase, WHO’s global database.
Signal Detection: Facilitates the analysis of safety signals by allowing researchers to group medications and investigate potential class effects.
Regulatory Submissions: Several regulatory bodies mandate or recommend its use for submitting drug data.

The Core Difference: Event vs. Substance

The fundamental difference between MedDRA and WHODrug is their specific focus. MedDRA answers the question, "What medical event happened to the patient?" For example, a patient in a clinical trial might report experiencing a 'headache' and 'nausea' after taking a study drug. These symptoms are medical events and would be coded using MedDRA.

Conversely, WHODrug answers the question, "What medicinal products did the patient take?" For the same patient, WHODrug would be used to code the name of the study drug, along with any other medications, such as a paracetamol for the headache. This separation of function is crucial. Analyzing safety data requires both pieces of information: the medical outcome (MedDRA) and the specific drug involved (WHODrug).

Comparison of MedDRA and Whodrug


Feature	MedDRA	WHODrug
Primary Purpose	To code medical events, symptoms, diagnoses, and procedures.	To code medicinal products, ingredients, and their classifications.
Subject Matter	Medical terminology (e.g., diseases, signs, symptoms).	Drug information (e.g., trade names, active ingredients, pharmaceutical form).
Classification System	Five-level, multi-axial hierarchy (LLT, PT, HLT, HLGT, SOC) based on medical concepts.	Anatomical Therapeutic Chemical (ATC) classification system based on pharmacological properties.
Maintainer	Maintenance and Support Services Organization (MSSO), under ICH.	Uppsala Monitoring Centre (UMC), a WHO Collaborating Centre.
Update Frequency	Bi-annually (March and September).	Quarterly (March, June, September, December).
Key Use Case	Adverse event reporting and safety signal detection.	Coding concomitant medications in clinical trials.

How MedDRA and Whodrug Work Together

In practice, MedDRA and WHODrug are not competing standards but are often used in tandem to provide a complete picture of a patient's safety profile. In clinical research and pharmacovigilance databases, these two dictionaries are integrated to link specific drugs (coded with WHODrug) to the medical events (coded with MedDRA) they may have caused. This integrated approach allows for comprehensive and accurate analysis of patient data, which is essential for effective safety surveillance and regulatory reporting. The synergy between these two powerful dictionaries is a cornerstone of modern drug safety practices.

Conclusion

While MedDRA and WHODrug both serve to standardize and code critical data in the pharmaceutical industry, they do so from different perspectives. The core difference between MedDRA and Whodrug is their object of focus: one describes the medical manifestations (events), while the other identifies the substances (drugs). Together, they provide the regulatory framework and analytical tools necessary for ensuring the safety and effectiveness of medicinal products globally. Understanding their separate yet complementary roles is fundamental for anyone involved in clinical trials, pharmacovigilance, and regulatory affairs.

For more in-depth information on WHODrug's structure and use, the Uppsala Monitoring Centre provides detailed resources, including their WHODrug Global overview.

Frequently Asked Questions

The primary purpose of MedDRA is to provide a standardized, international medical terminology for coding and analyzing medical events, including adverse reactions, symptoms, diagnoses, and patient medical history, for regulatory purposes.

The primary purpose of WHODrug is to provide a comprehensive drug dictionary for coding medicinal products, including their active ingredients, trade names, pharmaceutical form, and therapeutic classification.

No, MedDRA and WHODrug do not compete. They are complementary standards used together in pharmacovigilance and clinical research to provide a complete picture of drug safety, linking medical events to specific medications.

MedDRA is the dictionary used for coding adverse events and other medical occurrences. When an adverse event is reported, a MedDRA term is assigned to describe the medical concept.

WHODrug is the dictionary used for coding concomitant medications in clinical trials. It standardizes the names and ingredients of all drugs a patient is taking.

For many regulatory submissions, especially those involving clinical trial and post-marketing safety data, both MedDRA (for adverse events) and WHODrug (for medications) are mandated or required by authorities like the FDA and EMA.

MedDRA is maintained by the Maintenance and Support Services Organization (MSSO), while WHODrug is maintained by the Uppsala Monitoring Centre (UMC), a WHO Collaborating Centre.