> For the complete documentation index, see [llms.txt](https://docs.snomed.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.snomed.org/implementation-guides/drug-model-implementation-guide/content-development/drug-extension-development-principles.md).
# Drug Extension Development Principles
## Introduction
National drug extensions enable the representation of real-world medicinal products within the SNOMED CT drug model, complementing the international semantic backbone with country-specific content such as authorized products and packaged medicines.
The development of a drug extension requires adherence to a set of principles that ensure:
* alignment with the SNOMED CT concept model
* consistency across international and national content
* scalability through semi-automated processes
* interoperability with local drug dictionaries and regulatory sources
This chapter outlines the key principles for both overall extension strategy and authoring processes.
## Overall Principles
### Alignment with the International Model
Drug extensions are built on top of the SNOMED CT International Edition and must align with its concept model and hierarchy. The international model provides the core semantic structure through concepts such as:
* substances
* medicinal products (MP)
* medicinal product forms (MPF)
* clinical drugs (CD)
In some cases, national extensions may need to create concepts that logically belong to the international model, such as:
* new substances
* MPs, MPFs, or CDs not yet present
These concepts may subsequently be promoted to the International Edition following review by SNOMED International.
### Managing Concept Creation Across NRCs
The creation of international-level concepts by National Release Centers (NRCs) introduces coordination challenges, particularly where multiple countries may require the same concepts.
To reduce duplication and competition:
* SNOMED International is taking a more proactive role in creating substance concepts, for example by aligning with regulatory sources such as the European Medicines Agency (EMA)
* The creation of Medicinal Products (MPs) is also planned centrally
* Requests for MPFs and CDs can be submitted by NRCs and incorporated into the International Edition
This approach supports consistency, reduces duplication of effort, and ensures a shared foundation across extensions.
### Separation of International and National Responsibilities
A clear distinction should be maintained between:
* International content: abstract, reusable concepts supporting interoperability
* National extension content: real-world products, packages, and regulatory-specific information
National extensions should:
* reuse international concepts wherever possible
* only create new concepts when necessary
* ensure that extension concepts are logically aligned with the international model
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