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About FDA/USP Substance Registration System (SRS)


The primary goal of the FDA/USP Substance Registration System (SRS) is to unambiguously define all substances present in regulated products. Once a substance has been defined, the SRS assigns a strong identifier that is permanently associated with the substance. SRS is intended to be a general use resource and data presented here has been collected from public information sources. Inclusion of a substance in this database does not indicate that it is currently under review by the FDA or a substance in any FDA regulated product.


A substance is any matter that has a discrete existence, irrespective of origin, which may be biological or chemical. Substances can be single well-defined chemical entities containing a definite molecular structure, synthetic (e.g., isomeric mixtures) or naturally-occurring (e.g. conjugated estrogens) mixtures of chemicals, or materials derived from plants, animals, microorganisms or inorganic matrices that are not definable by a single or limited number of molecular structures. Substances may be active moieties, salts, solvates, stoichiometric complexes or mixtures of compounds that are either isolated or synthesized together. Materials that are combined from multiple sources to form a product are not considered substances. A substance is defined by what something is, and is not defined by how it is made or how it is used. Substance definitions are based on molecular structure or other immutable properties of a given material. Purity, physical form, and method of production are not considered when defining substances.

A UNII is an identifier for a defined substance. UNIIs contain 9 randomly generated alphanumeric characters with a tenth check alphanumeric character. In order to generate a UNII, a certain amount of information must be available to unambiguously identify a given substance. The assignment of the same UNII to two materials does not indicate that these materials are equivalent, but rather only indicates that they have the same elements upon which the definition is based. To request a UNII for the purpose of electronic listing via FDA's Structured Product Labeling (SPL), please contact SPL@fda.hhs.gov. For general information about SRS and UNIIs, please contact FDA-SRS@fda.hhs.gov.

Some ingredients will not have a UNII because they are comprised of multiple substances that are mixed together to form a product. Even if there are physical interactions between substances, they will not receive a UNII. For example, FD&C Blue No. 1 Aluminum Lake is not defined as a mixture substance but rather as two separate substances FD&C Blue No. 1 and aluminum oxide. Simethicone is not a substance because it contains two substances dimethicone and silicon dioxide. Conversely, if covalent bonds such as fluticasone propionate and/or simple salts or solvates are formed such as sodium succinate hexahydrate, separate UNIIs are assigned for each molecular entity (ie. succinic acid, sodium succinate, and sodium succinate hexahydrate).
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Last updated: Dec 2014

US Pharmacopeia