Methylisothiazolinone
INCI: METHYLISOTHIAZOLINONE
Reviewed by Kate · Last updated 14 Aug 2026
A synthetic preservative that prevents microbial growth in skincare formulations
Best For Skin Types
All
Addresses
Usage Frequency
As formulated in product
What Is Methylisothiazolinone?
Methylisothiazolinone is a synthetic preservative belonging to the isothiazolinone family. It works by inhibiting microbial growth, making it useful in cosmetic formulations to extend shelf life and prevent contamination. It is found in approximately 1.1% of skincare products.
As a preservative, its primary role is functional rather than therapeutic—it protects the formula itself rather than delivering a skincare benefit to the skin.
How It Works
Methylisothiazolinone acts as a biocide, meaning it inhibits the growth and reproduction of microorganisms such as bacteria and fungi. This prevents product degradation and contamination, which could otherwise compromise both safety and efficacy.
The ingredient's preservative action is concentration-dependent and can be influenced by other formula components, including surfactants, which may affect how readily it penetrates the skin barrier.
Clinical Evidence & References
Methylisothiazolinone is recognised as a potent contact allergen and has been subject to regulatory restrictions in the European Union due to concerns about allergic contact dermatitis and skin irritation. Research indicates that skin permeation varies depending on formulation conditions—particularly the presence of certain surfactants, which can enhance penetration.
Whilst its preservative efficacy is established, individual tolerance varies significantly. Those with sensitive skin, eczema, or a history of contact dermatitis may experience adverse reactions. The evidence suggests monitoring personal tolerance is important rather than assuming universal safety.
How to Use
If you have sensitive skin or a known sensitivity to isothiazolinone preservatives, patch test any new product containing this ingredient on a small area of skin first. Discontinue use if redness, itching, or irritation develops.
Safety & Precautions
Frequently Asked Questions
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