There are several alternative drug classification systems in addition to NDC that are also commonly used when analyzing drug data, such as Generic Product Identifier (GPI).
While the labeler code is assigned by the FDA, both the product and package segments are assigned by the labeler. While in the past labelers may have had the opportunity to reassign old product codes no longer used to new products, according to the new FDA validation procedures, once an NDC code is assigned to one product (defined by key properties including active ingredients, strength, and dosage form) it may not be later reassigned to a different product.
The following NDC structure rules apply depending on the length of certain segments. These rules include the 11 digit format. Unless stated otherwise, search for drug codes using the 10 digit format.
The official FDA format for NDCs separates the 3 segments with dashes. This is the format in which the NDC must be submitted by labelers since mandatory electronic listing was established in June 2009. Guidance for Industry - Providing Regulatory Submissions in Electronic Format – Drug Establishment Registration and Drug Listing
While the NDC is a 10-digit identifier, confusion exists because of a proliferation of different notations and variants.
The previous NDC directory, populated by a paper-based listing system, sometimes inserted an asterisk in either a product code or a package code in addition to the dashes. This format is easily transformed to the official format in effect today by deleting the asterisk (if any).
Each NDC-coded product package in the US bears a barcode with a Universal Product Code that begins with a 3 (UPC-A) or 03 (EAN-13). The remainder of the numbers can be the 10 NDC digits, plus the check digit, but this is not required. If the NDC is used in the UPC it is the most minimal form of the NDC code with 10 digits only. Because the NDC code has been linked with product barcodes in this way, the NDC code could contain ambiguities in this form. For example, 1234-5678-90, 12345-678-90, and 12345-6789-0 could all be entirely different products with the same barcode 1234567890. To prevent any actual ambiguity from impacting the marketplace, ambiguity checks are part of the new electronic listing process.
The pure 10-digit NDC format cannot be transformed back into the standard format with dashes without the help of the product listing database.
The Centers for Medicare and Medicaid Services (CMS) had created an 11-digit NDC derivative, which pads the labeler, product, or package code segments of the NDC with leading zeroes wherever they are needed to result in a fixed length 5-4-2 configuration (but always written without dashes). This format was adopted by data standards selected pursuant to HIPAA regulation, thus other government agencies' lists and databases (such as the UMLS) may contain the 11-digit derivative of the original NDC.
The 11-digit NDC format cannot be transformed back into the 10-digit standard format without the help of a product listing database.
In some applications, the fully expanded 5+4 digit labeler+product code is used as a 9-digit number to identify a product regardless of packaging. Thus an 11-digit NDC can be transformed into a 9-digit NDC product code by removing the last two digits. This does not work reliably for 10-digit NDCs where the packaging code might be one digit and not two.