Why is methylchloroisothiazolinone (MCI) banned from leave-on cosmetics?

Methylchloroisothiazolinone is a well-documented skin sensitizer and has been shown to trigger dermatitis at higher rates than most preservatives. The EU restricted it to rinse-off products only at very low concentrations (maximum 0.0015 percent) because the sensitization risk was deemed too high for leave-on exposure.

Is methylchloroisothiazolinone safe in rinse-off products?

At the EU-regulated maximum concentration of 0.0015 percent in rinse-off products, available safety assessments consider it acceptable for this limited use. However, people with reactive or compromised skin should patch test or avoid products containing this preservative, as it remains a known irritant and sensitizer.

What should I use instead of methylchloroisothiazolinone?

Common alternatives include phenoxyethanol, sodium benzoate, and other gentler preservatives. Your cosmetics brand can recommend swaps based on their formulation needs and stability requirements.

preservative

Methylchloroisothiazolinone

Restricted·Strong evidence

Methylchloroisothiazolinone is typically used together with methylisothiazolinone in a 3:1 mix and is a well-documented skin sensitizer at higher rates than most preservatives. EU rules now restrict this combination to rinse-off products only, at a low maximum concentration.

Worth knowing

Known contact sensitizer for some
Can irritate, especially left on
Restricted in the EU

EU permits the MCI/MIT (3:1) mixture only in rinse-off products at a maximum of 0.0015% (15 ppm); banned from leave-on products since 2017.

Gentler alternatives

Questions

Why is methylchloroisothiazolinone (MCI) banned from leave-on cosmetics?: Methylchloroisothiazolinone is a well-documented skin sensitizer and has been shown to trigger dermatitis at higher rates than most preservatives. The EU restricted it to rinse-off products only at very low concentrations (maximum 0.0015 percent) because the sensitization risk was deemed too high for leave-on exposure.
Is methylchloroisothiazolinone safe in rinse-off products?: At the EU-regulated maximum concentration of 0.0015 percent in rinse-off products, available safety assessments consider it acceptable for this limited use. However, people with reactive or compromised skin should patch test or avoid products containing this preservative, as it remains a known irritant and sensitizer.
What should I use instead of methylchloroisothiazolinone?: Common alternatives include phenoxyethanol, sodium benzoate, and other gentler preservatives. Your cosmetics brand can recommend swaps based on their formulation needs and stability requirements.

Products with Methylchloroisothiazolinone

In our guides

Ingredients
Which preservatives are actually worth watching

References

Cosmetic information for general education, not medical advice. A verdict is a reading of the published evidence, never a guarantee for your skin: any ingredient can irritate someone, so patch test new products and see a professional if you react. Concern is graded on cited evidence, never on hazard-score lists. See how we score.