Methylchloroisothiazolinone and Methylisothiazolinone (MCI/MI): the EU Rules and Why They Changed

Methylchloroisothiazolinone (MCI) and methylisothiazolinone (MI) are two isothiazolinone preservatives that spent two decades as one of the most common preservative systems in rinse-off cosmetics, then became one of the most heavily restricted. This article sets out what the two substances are, exactly what the EU Cosmetics Regulation allows, which acts changed the rules and when, how a handful of other markets in cosingchecker.com's data treat the same CAS numbers, and what to check on a real label.

Identities and CAS numbers

Three separate substance identities are involved, and mixing them up is the most common source of confusion when checking methylchloroisothiazolinone products against the rules.

These are drawn from cosingchecker.com's own ingredient records for methylchloroisothiazolinone (cosingchecker.com/ingredients/35333-methylchloroisothiazolinone/) and methylisothiazolinone (cosingchecker.com/ingredients/35341-methylisothiazolinone/), and from the two Annex V entries themselves. The mixture is sold commercially as Kathon CG; MI alone is sold as, among other trade names, Neolone 950. Trade names are not part of the legal text and are not a reliable way to identify the substance on an ingredient list — the CAS or EC number, or the INCI name, is.

The two Annex V entries

Annex V to Regulation (EC) No 1223/2009 lists preservatives allowed in cosmetic products subject to conditions. Two separate entries cover these substances, and they are not interchangeable.

• Entry 39: the mixture of MCI and MI in a fixed 3:1 ratio. Maximum concentration 0.0015% of the mixture, rinse-off products only.
• Entry 57: MI on its own (not combined with MCI). Maximum concentration 0.0015%, rinse-off products only.

Both entries carry the identical 0.0015% (15 ppm) ceiling and the identical rinse-off-only scope, but they are legally distinct: a formulator using the pre-mixed 3:1 blend must comply with entry 39, and one adding MI alone must comply with entry 57. Because the two share a component (MI), a product cannot combine an entry-57 dose of standalone MI with an entry-39 dose of the mixture and still claim both are within their individual limits — the cumulative MI exposure is what the restriction is trying to cap. cosingchecker.com's Annex V entry pages record this at cosingchecker.com/annex/V/v-39-methylchloroisothiazolinone-and-methylisothiazolinone/ and cosingchecker.com/annex/V/v-57-methylisothiazolinone/, and the general guide to reading an Annex V preservative entry is at cosingchecker.com/blogs/eu-annex-v-preservatives-how-to-read-an-entry/.

What "rinse-off" and "leave-on" mean in the annex

Annex V does not define these terms substance by substance; it uses them as product-type qualifiers attached to a given entry's conditions field. "Rinse-off" means the product is intended to be removed from the skin or hair shortly after application — shampoo, shower gel, liquid soap. "Leave-on" covers everything meant to stay on the skin — lotions, creams, sunscreens, wet wipes used without rinsing. Because both entry 39 and entry 57 restrict MCI and MI to rinse-off use only, neither substance currently has any permitted leave-on use under Annex V at any concentration. A product that lists MCI, MI, or the combined name on a leave-on formulation's ingredient list is not compliant with either entry.

Why the rules changed: the contact-allergy record

MCI/MI's Annex V history goes back to Directive 76/768/EEC, where the 3:1 mixture was authorised at 0.0015% in all cosmetic products, rinse-off and leave-on alike, with the SCCP (the Scientific Committee's predecessor body) reviewing its safety more than once as clinical patch-test data accumulated. The Scientific Committee on Consumer Safety (SCCS) tightened the mixture's terms with an opinion that fed into Commission Regulation (EU) No 1003/2014, which amended entry 39 to remove leave-on use, keeping the same 0.0015% ceiling for rinse-off products only, with compliant products required on the market from 16 July 2015 and non-compliant stock withdrawn by 16 April 2016.

MI alone was, at that point, still permitted in both rinse-off and leave-on products at up to 0.01% (100 ppm) under a separate, older entry. Rising numbers of contact-allergy cases attributed to MI in leave-on products (including wet wipes) led the SCCS to conclude, in its opinions on methylisothiazolinone (sensitisation only) of December 2013 (revised March 2014) and December 2015, that no concentration of MI in leave-on cosmetics had been shown to be safe from the point of view of induction of contact allergy, while 0.0015% in rinse-off products was considered safe. That finding was implemented in two steps: Commission Regulation (EU) 2016/1198 removed MI's leave-on permission entirely, with only compliant products allowed on the market from 12 February 2017; Commission Regulation (EU) 2017/1224 then replaced the rinse-off entry with the current entry 57, cutting the rinse-off ceiling from 0.01% to 0.0015%, with compliant-only placement on the market from 27 January 2018 and compliant-only availability from 27 April 2018. The SCCS opinions behind these changes are listed on the two Annex V entry pages.

How other jurisdictions treat the same substances

cosingchecker.com's market-compare view (cosingchecker.com/market-compare/) and the per-ingredient jurisdiction endpoints let you see how markets outside the EU treat the same CAS numbers. Coverage varies by market and reflects what each regulator has published, not a claim that unlisted markets allow unrestricted use.

The pattern across these records is consistent with the EU's own conclusion: every market with a published position caps both substances to rinse-off use at or near 0.0015%, several (Canada, South Korea, Australia) explicitly track the combined MI exposure across both entries the way EU practice implies, and none of the markets in the site's data currently shows a leave-on permission for either substance. Where a jurisdiction's rules page does not mention MCI or MI at all, that is an absence from the published data cosingchecker.com has recorded, not a statement that the substances are unrestricted there — check the regulator's own source for anything not covered.

What to check on a product label

• Read the full ingredient list, not just the marketing claims. MCI/MI can appear as "Methylchloroisothiazolinone (and) Methylisothiazolinone" (the 3:1 mixture, entry 39) or as "Methylisothiazolinone" alone (entry 57) — these are different entries with different legal bases even though the concentration cap is the same number.
• Match the product type to the entry's scope. If a leave-on product (a cream, lotion, or sunscreen) lists either name, that is inconsistent with the current Annex V permissions, which cover rinse-off use only.
• Do not assume trade names. "Kathon CG" and similar proprietary names are not required on the label and are not a substitute for the INCI name when checking compliance.
• Remember a database record shows what is listed, not a verdict. cosingchecker.com's ingredient and Annex V pages report what the EU (or another jurisdiction) has published; they do not certify that any specific product complies, and the underlying legal text at EUR-Lex or the relevant regulator is the authority for a compliance decision.

For the mechanics of reading any Annex V preservative entry field by field, see cosingchecker.com/blogs/eu-annex-v-preservatives-how-to-read-an-entry/. For finding an entry by its number across annexes, see cosingchecker.com/blogs/how-to-find-an-eu-annex-entry-by-its-number/.

Sources

• Commission Regulation (EU) No 1003/2014 amending Annex V to Regulation (EC) No 1223/2009 — https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32014R1003
• Commission Regulation (EU) 2016/1198 amending Annex V to Regulation (EC) No 1223/2009 (methylisothiazolinone) — https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32016R1198
• Commission Regulation (EU) 2017/1224 amending Annex V to Regulation (EC) No 1223/2009 (methylisothiazolinone) — https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32017R1224
• Regulation (EC) No 1223/2009 of the European Parliament and of the Council on cosmetic products — https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32009R1223
• cosingchecker.com Annex V entry 39 (MCI and MI mixture) — https://cosingchecker.com/annex/V/v-39-methylchloroisothiazolinone-and-methylisothiazolinone/
• cosingchecker.com Annex V entry 57 (methylisothiazolinone) — https://cosingchecker.com/annex/V/v-57-methylisothiazolinone/
• cosingchecker.com ingredient record: methylchloroisothiazolinone — https://cosingchecker.com/ingredients/35333-methylchloroisothiazolinone/
• cosingchecker.com ingredient record: methylisothiazolinone — https://cosingchecker.com/ingredients/35341-methylisothiazolinone/
• cosingchecker.com market-compare — https://cosingchecker.com/market-compare/