CosmeticBet
Preservatives

Methylchloroisothiazolinone

INCI: METHYLCHLOROISOTHIAZOLINONE · also MCI, MCI/MI

MI’s partner in a strong preservative mix.

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C4H4ClNOS149.60 g/mol
chlorinesulfuroxygennitrogencarbonhydrogen
3D shape from PubChem 33344

How common

All products3.5%
In conditioners21%
In shampoos15%
In hair colours14%

520 of 14,784 products with a full ingredient list in Open Beauty Facts; No. 88 of all the names found on their labels.

What it is

  • A preservative used as a 3-to-1 mixture with methylisothiazolinone (MI).

    2 sourcesHide
    the mixture Methylchloroisothiazolinone (MCI)/Methylisothiazolinone (MI), which functions as a preservative in cosmetic products[1] US Cosmetic Ingredient Review expert panel · 2021 · open source
    (of a mixture in the ratio 3:1 of 5-chloro-2-methylisothiazol 3(2H)-one and 2-methylisothiazol-3 (2H)-one)[2] EU law · 2026 · open source

Why it is in products

  • To stop microbes growing in the product.

    SourceHide
    ‘preservatives’ means substances which are exclusively or mainly intended to inhibit the development of micro-organisms in the cosmetic product[2] EU law · 2026 · open source

Downsides and safety

  • An important skin allergen: 2.33% of patients patch tested in Germany from 1992 to 2010 reacted to it.

    Well established3 sourcesHide
    is an important allergen[3] Study · 2012 · open source
    The overall frequency of positive patch test reactions was 2.33%[3] Study · 2012 · open source
    1992-2010[3] Study · 2012 · open source

Where it works

Works insidethe product

It works in the product; the allergy, when it happens, is in the skin.

SourceHide
‘preservatives’ means substances which are exclusively or mainly intended to inhibit the development of micro-organisms in the cosmetic product[2] EU law · 2026 · open source

Rules

European Union

  • Only in rinse-off products, at most 0.0015% of the mix (Annex V, entry 39).

    SourceHide
    Methylchloroisothiazolinone (and) Methylisothiazolinone ( 17 ) 26172-55-4, 2682-20-4, 55965-84-9 247-500-7, 220-239-6 Rinse-off products 0,0015 %[2] EU law · 2026 · open source

United States

  • Not on the FDA’s list of prohibited or restricted cosmetic ingredients. In the US, cosmetics and their ingredients (other than colour additives) don’t need FDA approval before sale.

    SourceHide
    Under U.S. law, cosmetic products and ingredients, other than color additives, do not need FDA approval before they go on the market[4] US Food and Drug Administration · open source
    We searched the FDA’s list of prohibited and restricted cosmetic ingredients: no entry.open the list

  • The US expert panel: safe when formulated not to sensitise, never above 7.5 ppm in leave-on or 15 ppm in rinse-off products.

    SourceHide
    at no point should concentrations exceed 7.5 ppm in leave-on products or 15 ppm in rinse-off products[1] US Cosmetic Ingredient Review expert panel · 2021 · open source

Sources

  1. Amended Safety Assessment of Methylchloroisothiazolinone and Methylisothiazolinone as Used in Cosmetics International journal of toxicology, 2021US Cosmetic Ingredient Review expert panel
  2. Regulation (EC) No 1223/2009 on cosmetic products, consolidated text of 18 May 2026 Publications Office of the European Union (EUR-Lex/Cellar), 2026EU law
  3. Methylchloroisothiazolinone/methylisothiazolinone contact sensitization: diverging trends in subgroups of IVDK patients in a period of 19 years Uter W et al., Contact dermatitis, 2012Study
  4. Prohibited & Restricted Ingredients in Cosmetics | FDA US Food and Drug Administration

Checked against these sources on 4 November 2026. Spot a mistake? Tell us.