Why does CAS 51-21-8 appear in more than one EU cosmetic annex?

A substance identified by CAS 51-21-8 may appear in multiple EU cosmetic regulation annexes when it is subject to different conditions in different contexts — for example, restricted under Annex III with specific conditions, while also appearing in Annex IV as an approved colorant under separate entry conditions. Each entry must be reviewed individually.

How do I check if my formula contains a substance with CAS 51-21-8?

Use the INCI Checker on CosIng Checker with INCI names, CAS numbers or EC numbers. Paste CAS 51-21-8 to screen it directly, then review every linked annex record and supplier document.

CAS 1 record

CAS 51-21-8

Q: How do I use CAS 51-21-8 for compliance screening?

Cross-reference CAS 51-21-8 against your raw material specification sheets and CoA documents. When it appears in Annex II, verify absence. When it appears in Annex III, verify that concentration limits and product type conditions are met. All annex entries sharing this CAS number are listed on this page.

CAS 51-21-8 matches 1 EU annex record across Annex II–VI of Regulation 1223/2009.

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Chemical identity and properties

PubChem 2D chemical structure for CAS 51-21-8

CAS 51-21-8PubChem CID 3385

5-Fluorouracil

5-fluorouracil is a nucleobase analogue that is uracil in which the hydrogen at position 5 is replaced by fluorine. It is an antineoplastic agent which acts as an antimetabolite - following conversion to the active deoxynucleotide, it inhibits DNA synthesis (by blocking the conversion of deoxyuridylic acid to thymidylic acid by the cellular enzyme thymidylate synthetase) and so slows tumour growth. It has a role as a radiosensitizing agent, an antimetabolite, an antineoplastic agent, a xenobiotic, an immunosuppressive agent and an environmental contaminant. It is an organofluorine compound and a nucleobase analogue. It is functionally related to a uracil. — ChEBI

IUPAC name: 5-fluoro-1H-pyrimidine-2,4-dione

Molecular formula: C4H3FN2O2
Molecular weight: 130.08 g/mol
Exact mass: 130.01785550 Da
XLogP3: -0.9
Topological polar surface area: 58.2 Å²
Hydrogen-bond donors: 2
Hydrogen-bond acceptors: 3
Covalent units: 1
Defined stereocentres: 0

Also known as

fluorouracilEfudexFluracilFluoroblastinCarzonalQueroplexFlurilPhthoruracil

16 more reported names

ArumelTimazinFluracilumUlup5-FluoracilFluorouracilumFluro Uracil5 Fluorouracil5-Fluoruracil5-Fluoro-2,4(1H,3H)-pyrimidinedioneFluorouraciloFtoruracilEfurixEffluderm (free base)Fluroblastin5-Fluoropyrimidine-2,4-dione

Open full PubChem record PubChem annotations 3D structure image

External database identifiers

ChEBI:CHEBI:46345
ChEMBL:CHEMBL185
EPA DTXSID:DTXSID2020634
PubMed:98292442
PubMed:40852754
PubMed:40846140
PubMed:40613580
PubMed:40383470
PubMed:40239743
PubMed:40228806
PubMed:40158757
PubMed:39922517
PubMed:39843995
PubMed:39667421
PubMed:39334374

Retrieved from NIH PubChem on 2026-08-31 by checksum-valid CAS matching. Chemical properties do not replace the regulatory decision on this page.

Records for CAS 51-21-8

1 total

Annex

Entry

190

Fluorouracil (INN)

200-085-6

How CAS 51-21-8 is used on this site

CAS 51-21-8 is the Chemical Abstracts Service identifier used to pin this page to a specific substance. In the current dataset, 1 EU annex record shares this identifier, which helps you cross-check whether the same substance appears under one regulatory context or several.

Use this page when a supplier specification, SDS or raw-material dossier gives you CAS 51-21-8 and you need to see every linked Annex II–VI record in one place. The entry cards above are the quickest route to concentration limits, warnings, annex placement and product-type conditions.

Frequently asked questions

A substance identified by CAS 51-21-8 may appear in multiple EU cosmetic regulation annexes when it is subject to different conditions in different contexts. For example, a substance might be restricted under Annex III with specific conditions, and the same CAS might also appear in Annex IV as an approved colorant under separate entry conditions. Each entry must be reviewed individually.

Cross-reference the CAS number against your raw material specification sheets and CoA documents. When a CAS appears in Annex II, verify absence. When it appears in Annex III, verify that concentration limits and product type conditions are met in your formulation. All annex entries sharing this CAS number are listed above.

A CAS number is assigned by the Chemical Abstracts Service (USA) and is globally recognised. An EC number is a European Commission identifier from inventories such as EINECS, ELINCS or the NLP list. Both identifiers may appear in EU cosmetic annex entries. Some substances have both, some have only one, and some older entries may have neither.

The INCI Checker accepts ingredient names, CAS numbers and EC numbers. Check this CAS number directly, then verify the result against supplier specification sheets and every linked annex condition.

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Tools & next steps

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