Definition

Unique Ingredient Identifier (UNII)

A Unique Ingredient Identifier (UNII) is a free, non-proprietary 10-character alphanumeric code that the FDA assigns to a substance, such as an active or inactive drug ingredient, based on its scientific identity.

1 min readReviewed September 14, 2026

Also known as: UNII code, FDA UNII, FDA substance identifier

Key facts

Assigned by
FDA, through the Global Substance Registration System (GSRS)
Format
10 alphanumeric characters with no embedded meaning
Cost
Free and non-proprietary
Data model
Based on ISO 11238, the IDMP standard for substances
Covers
Substances in FDA-regulated products, including drugs, biologics and foods

What is a UNII?

Every ingredient in an FDA-regulated product needs an unambiguous identity, but ingredient names vary between companies, countries and databases. The Unique Ingredient Identifier (UNII) solves this by assigning one code per substance, based on what the substance is, such as its molecular structure or other defining description, rather than on its name.

UNIIs are generated by the Global Substance Registration System (GSRS), developed by the FDA with the National Center for Advancing Translational Sciences (NCATS) at the National Institutes of Health. The codes are free and carry no meaning in themselves: a UNII cannot be decoded to learn anything about the substance.

How UNIIs are assigned

Assignment follows the substance, not the product:

The GSRS data model follows ISO 11238, the substance standard within the Identification of Medicinal Products (IDMP) family of standards.

  • Chemicals with a defined structure are identified by that structure, so different salts or hydrates of a drug receive different UNIIs from the parent molecule.
  • Proteins, nucleic acids, polymers, mixtures and materials such as plant extracts are defined by other descriptive information.
  • A UNII belongs to a substance, so one UNII can appear in thousands of products from many labelers.
  • Inactive ingredients, such as excipients and colorants, receive UNIIs just as active ingredients do.

Why UNIIs matter

UNIIs are the most reliable ingredient key in FDA drug data:

  • Structured product labeling (SPL) files identify each active and inactive ingredient by UNII, so label and National Drug Code (NDC) data can be grouped by ingredient.
  • Matching by UNII avoids errors from spelling differences and synonyms in ingredient names across datasets.
  • Separate UNIIs for salt forms let analysts decide deliberately whether to group products by exact substance or by the underlying molecule.
  • openFDA includes UNIIs in its harmonized fields, which helps link adverse event, recall and labeling records to a substance.

Sources

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