NDC (National Drug Codes) is a coding system used for drugs commercially available in the US. NDC consist of three components:

1. manufacturer number (up to 5 digits)
2. product code (up to 4 digits)
3. packaging information (up to 2 digits)

The FDA assigns the manufacturer number to each manufacturer. Manufacturers can be organizations that actually manufacture the drug, resellers or distributors. Each manufacturer is responsible for encoding the product and packaging information. This system is not strictly enforced, which means that NDC codes with the same manufacturer number and product code do not always designate the same product. It also means that NDC codes can be repeated over time designating different products, though this happens very rarely (also dubbed code re-use).

NDCs are also reported in different format variants. Where possible OHDSI adopts the standardized 11-digits HIPAA format used by the NLM and described in Section 6 of the RxNorm documentation. Depending on the source the 9-digit format is used otherwise.

There is not one comprehensive repository of all NDC codes available. Instead, the FDA, the NLM and a number of commercial providers distribute NDC code lists. OHDSI is trying to create a list as complete as possible by combining sources from the official FDA NDC code distribution web site and the files that can be retrieved for SPL (Standard Product Labeling) concepts, as well as occasionally adding NDC codes found in observational databases manually. If NDC codes are missing from the CONCEPT table, please report those to the OHDSI forum.

This however creates some limitations around accuracy and completeness of the NDC vocabulary that cannot be easily overcome. In addition, many NDC concepts lack a mapping to a Standard RxNorm Drug concept. These are available for those NDCs that come with a RxNorm release, or for which a heuristic can be establish to create the mapping indirectly. Fortunately, most of these non-mapped concepts are not real drugs, but devices and all sorts of medical supplies.

The procedures for transforming concepts from the source format to the OMOP Standard Vocabularies can be found on the OHDSI GitHub.

All Concepts are assigned the Full Source Name.

Concept codes are the 9 or 11 digit NDC codes without dashes.

• All Drug domain concepts are non-standard.
• All Devices are designated as standard concepts.

The majority of NDC concepts are in the “Drug” domain.

Domain	Notes
Drug	Medication with an actual active therapeutic ingredient
Device	Medical devices or drugs with only diagnostic ingredients (e.g. contrast media)

The NDC concepts are composed from the following Concept Classes:

Class	Notes
11-digit NDC	Format determined by source data provider. Concept with a full HIPAA compliant 11 digit code without dashes
9-digit NDC	Format determined by source data provider, omitting the 2-digit packaging information.
Device	Devices as defined by the OHDSI conventions for Devices and “Drug-Devices”

Drug related NDC Concepts are non-Standard Concepts and therefore do not participate in the hierarchy of the CONCEPT_ANCESTOR table. The Device standard concepts stand alone and are not integrated into a hierarchy.

A considerable number of concepts is non-standard but has no mapping to standard concepts (above 200K). These are entries where no mapping information has been provided and which are poorly accessible to other automated mapping approaches.

Most NDC concepts are non-Standard. That means they have to be mapped to the corresponding Standard Concepts using the CONCEPT_RELATIONSHIP table ("Maps to" and occasionally "Maps to value" records). Most of them are mapped to single RxNorm Concepts, generating one-to-one records in the drug_exposure table, but some of them create multiple records (multi ingredient drugs are mapped to all their ingredients individually) or have mappings to other domains (as in devices).