20 May 2019

Updated 2 Nov 2021

The MedDRA and WHODrug Dictionaries: What Are They and How Are They Used?


The MedDRA and WHODrug Dictionaries: What Are They and How Are They Used?

Using medical dictionaries to code clinical trial data is a key way to ensure that your clinical trial records and databases are easy to interpret and analyze. Coding means taking the free text entered into electronic case report files (eCRFs) and mapping it to one or more entries in a medical dictionary.  Two of the most widely used medical dictionaries in clinical trial systems are MedDRA and WHODrug Global.

What is MedDRA?

MedDRA, the Medical Dictionary for Regulatory Activities, is a multilingual dictionary of standard terminology that is used to code medical events in humans, including patients’ medical histories and adverse events. The MedDRA dictionary is based on a five-tier hierarchical structure (Figure 1) that goes from system organ class (e.g. gastrointestinal disorders) down to lowest level terms (e.g. feeling queasy).  This dictionary was originally based on a set of terminology used in the United Kingdom in the 1990s, and the ICH (the International Council for Harmonisation of Technical Requirements for Pharmaceuticals for Human Use) spearheaded the effort to bring this standard set of medical coding terms into use worldwide. MedDRA is maintained, distributed, and supported by the MSSO (MedDRA Maintenance and Support Services Organization) and the JMO (Japanese Maintenance Organization). MedDRA has undergone multiple revisions since it first began, and is updated every six months.

What is WHODrug Global?

WHODrug Global is a drug reference dictionary that was first created in 1968, and uses the Anatomical Therapeutic Chemical classification system to classify drugs. WHODrug Global contains codes for a wide range of drugs, and is available in different scopes such as WHODrug Enhanced and WHODrug Herbal, which enable coding of everything from conventional medicines to herbal treatments. Using WHODrug Global portfolio coding includes a variety of tools and resources that can be used to help protect patients by clearly classifying their responses to different treatments, thereby improving analysis of effectiveness and safety. This medical coding dictionary is maintained and supported by the Uppsala Monitoring Center.

Challenges in Using the Dictionaries 

Overall, using the MedDRA and WHODrug Global dictionaries for medical coding in clinical trial systems makes it much easier to record and analyze patient data in a consistent and accessible way.

That being said, there are some challenges associated with using both of these medical coding systems in CTMSs. For example, different WHODrug Global identifiers may apply when a single drug is used for different indications. In MedDRA, classification codes are so detailed that, in some cases, accurate coding may unintentionally mask some adverse events. There is also the problem of consistency between different users of the same clinical trial system: a study reported that, in the same database, “12% of the codes were evaluated differently by two coders… [and] 8% of all the codes deviated from the original description”.

Integration of Dictionaries in EDC Systems

As you can imagine, MedDRA and WHODrug Global integration into clinical trial systems go a long way to making patient data easier to record, analyze, and report with patient data and coding data being at the same place. Top electronic data capture (EDC) systems have integrated search engines for all MedDRA terms and WHODrug Global elements, which makes it quick and easy to lookup dictionary terms and enter them directly into eCRFs. Some systems can learn to recognize study-specific verbatim terms, for example by using a manually coded synonym list to then automatically code these terms the same way that they were in earlier entries, which makes the data more consistent and easier to interpret. Another important advantage of using an EDC with integrated MedDRA and WHODrug Global dictionaries is that it can be used to download and implement updates to these coding systems as needed.

In addition to simply recording patient data and outcomes in a consistent, searchable way, using standardized medical coding dictionaries can enable the identification of adverse events in near real time. This facilitates adverse event reporting, and, if needed, provides a timely alert for severe adverse events and suspected unexpected serious adverse reactions that may necessitate pausing or halting the trial as expected by authorities.

Image Credit: Alexander Raths /

Get a Demo

Let us show you how we can help you.

Get a Quote

Discover a solution tailored to your needs and budget.

Get a Meeting

Let’s start a new project together!

Let’s Start
a conversation

Ready to discover how our platform and its suite of applications can meet your needs?


    We use cookies to personalize content and ads, provide social media features, and analyze traffic to our website. Our partners may combine this information with other data they have provided to you or collected from your use of the services. Because we value your privacy, we hereby request your permission to use the following technologies. You may change / revoke your consent later at any time by clicking on the settings in the lower left corner of the page.

    Please note that according to a recent ruling by the European Court of Justice (ECJ), there is no adequate level of data protection in the USA and therefore a risk to the protection of your data. For example, under certain conditions, your data may be processed by US authorities for control and monitoring purposes. In addition, we currently refer to Art. 49 GDPR with regard to the legal basis for the transfer of data. After implementation of the new EU standard data protection clauses, these will constitute the legal basis for data transfer to third countries.