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Methylchloroisothiazolinone

Also known as: MCI, CMIT, Chloromethylisothiazolinone, MCI/MI

A highly efficient preservative usually paired with methylisothiazolinone and closely watched for contact allergy.

Amber formulation vial and clear droplets in an archival paper composition
Editorial illustration

Methylchloroisothiazolinone (MCI or CMIT) is a potent cosmetic preservative usually used in a mixture with methylisothiazolinone. It controls microbes at low concentrations, but the mixture is a recognized contact allergen. Rinse-off exposure and leave-on exposure should not be treated as equivalent.2, 3, 1

  • Usually appears as an MCI/MI preservative mixture1, 2
  • Works at very low concentrations1, 2
  • Contact sensitization is well documented1, 2
  • Product format and regional limits matter1, 2

What is Methylchloroisothiazolinone?

MCI is an isothiazolinone biocide used to protect water-based products. On an ingredient list it often sits beside methylisothiazolinone because the commercial preservation system contains both molecules.1, 2

Where it comes from

Natural sources

  • Not naturally harvested

How it's made

Manufactured as a defined isothiazolinone preservative, commonly supplied as a controlled MCI/MI blend.

What it does in products

  • Preservative
  • Bacterial control
  • Fungal control

Benefits

Low-level preservation

moderate evidence

MCI/MI can control microbes at much lower use levels than many traditional preservative systems.2, 1

Preservation efficacy is established, but efficacy does not erase sensitization risk.

Protection of rinse-off products

moderate evidence

The preservative helps keep products such as shampoo microbiologically stable during repeated use.2

Safety conclusions depend on the permitted product type, concentration, and exposure pattern.

Suitability for eczema-prone users

insufficient evidence

A history of dermatitis does not prove MCI allergy, but sensitized users need strict avoidance.3

Patch testing, not a generic sensitive-skin label, establishes contact allergy.

Evidence scope and limitations

Cosmetic MCI/MI preservation and consumer skin contact are covered; industrial biocide exposures are not generalized to finished cosmetics.

  • Sensitization prevalence changes with regulation and use patterns.
  • Mixture data cannot always isolate MCI from MI.

Evidence last checked: 2026-09-28

Side effects & safety

Allergic contact dermatitis (uncommon)A well-recognized preservative allergy.
Irritant dermatitis (uncommon)Risk depends on dose, contact time, and skin condition.

Safety considerations

  • No expected drug interaction from compliant cosmetic exposure
  • Not for: Confirmed MCI/MI or isothiazolinone allergy
  • Not for: Continued use after a reproducible rash
2, 3

Amounts studied & product use

These figures describe research or common product use, not a personal dose or treatment recommendation.

In skincare studies and products

MCI/MI levels are controlled at the formulation level and differ by jurisdiction and rinse-off status. It is not a DIY preservative and should not be added directly to a product.

2

Regulatory status

  • Used as a cosmetic preservative in the United States, where the manufacturer is responsible for safety; other markets apply product-type restrictions.
  • INCI name on cosmetic labels: Methylchloroisothiazolinone.
  • CIR has reviewed MCI together with methylisothiazolinone with emphasis on sensitization and use conditions.
4, 2

Why rinse-off versus leave-on matters

A shampoo that is diluted and rinsed has a different exposure pattern from a cream left on skin all day. Regulations and safety assessments distinguish those scenarios; an ingredient-list screenshot without product type leaves out essential context.

2, 3

Frequently asked questions

Is MCI the same as MI?

No. They are related preservatives and are often supplied together, so both may appear on the label.2, 3, 1

Can MCI cause eczema?

It can cause allergic contact dermatitis in sensitized people; patch testing can clarify the allergen.2, 3, 1

Why is it still used?

It provides strong microbial control at low levels, especially in rinse-off products, but use conditions matter.2, 3, 1

The bottom line

MCI is effective preservation chemistry with a real allergy limitation; exposure format and personal sensitization decide more than fear-based ingredient lists.

References

  1. Methylchloroisothiazolinone: compound summary. PubChem, National Library of Medicine (2026) · ingredient monograph · CID 33344
  2. Amended safety assessment of methylisothiazolinone and methylchloroisothiazolinone as used in cosmetics. Cosmetic Ingredient Review (2019) · safety assessment
  3. Isothiazolinone derivatives and allergic contact dermatitis: a review and update. Journal of the European Academy of Dermatology and Venereology (2019) · other · PMID 30284765
  4. FDA Authority Over Cosmetics: How Cosmetics Are Not FDA-Approved, but Are FDA-Regulated. U.S. Food and Drug Administration (2013) · government guidance

Revision history

  1. Initial publication with claim-level citations, formulation limits, safety context, and editorial evidence review.