Ingredient review

Methylchloroisothiazolinone

INCI: METHYLCHLOROISOTHIAZOLINONE

A powerful preservative that is a common contact allergen; best avoided by those with sensitive skin.

beautyskincarepreservative

In plain English

Methylchloroisothiazolinone (MCI) is a synthetic preservative that stops bacteria, yeast, and mold from growing in water-based products. It is very effective at very low levels, but it is also one of the most common causes of allergic contact dermatitis. For this reason, it is mostly restricted to rinse-off products like shampoos and body washes, and is rarely found in leave-on creams or lotions.

Review score

Safety, usefulness, and evidence

2.5Fair

2.5/ 5

Higher-caution ingredient

The evidence base is relatively strong for its common cosmetic role.

Main practical flags: irritation is high; clogging risk is low.

Source: synthetic
Evidence: strong
Irritation: high
Clogging risk: low

How reviews are scored

Quick decision guide

Read the cautions before using

Methylchloroisothiazolinone can be useful, but watch for high irritation potential.

Plain-English read

Treat this as a practical screening step before you compare products that contain this ingredient.

Step 1Start with the score, then check the irritation and clogging risk before judging Methylchloroisothiazolinone.
Step 2Use the "Best for" and "Use caution if" sections to match the ingredient to your skin, not just to a marketing claim.
Step 3If a product stings, breaks you out, or worsens irritation, judge the finished formula and stop using it even if the ingredient scores well.

Score terms in plain English

Irritation risk

high

More likely to cause dryness, stinging, peeling, or reactivity if used too aggressively.

Clogging risk

low

Less likely to feel heavy or contribute to clogged pores for most skin types.

Evidence level

strong

There is a stronger practical or research basis for the ingredient role described here.

What it is

Methylchloroisothiazolinone is a synthetic organic compound that belongs to the isothiazolinone family. It is almost always used in combination with methylisothiazolinone (MI) in a 3:1 ratio, a mixture known as Kathon CG. It acts as a broad-spectrum preservative.

How it works

MCI works by disrupting the cell membranes and metabolic processes of microorganisms. It penetrates the cell wall of bacteria and fungi, interfering with their ability to grow and reproduce, thereby preserving the product from spoilage.

Pros

Highly effective preservative

Works at very low concentrations (0.0015–0.01%) to prevent microbial contamination, keeping products safe for use.

Broad-spectrum activity

Kills bacteria, yeast, and mold, which means formulators can use it as a single preservative system in many rinse-off formulas.

Cons and cautions

Common allergen

MCI is one of the most frequent causes of allergic contact dermatitis, especially in leave-on products. Reactions can include redness, itching, and blistering.

Skin irritation risk

Even in people without a true allergy, MCI can cause irritation, stinging, or dryness, particularly on compromised or sensitive skin.

Best for

People with normal, non-sensitive skin using rinse-off products
Those who tolerate standard preservatives without reaction

Use caution if

Anyone with a history of contact dermatitis or skin allergies
People with eczema or very sensitive skin
Those using leave-on products (creams, lotions, serums)

Usage tips

Only use products containing MCI if they are rinse-off formulas like shampoo or body wash.

If you have sensitive skin or a history of allergies, choose products labeled 'preservative-free' or preserved with alternatives like phenoxyethanol.

Patch test new products on a small area of skin before full use if you are concerned about a reaction.

Safety summary

Methylchloroisothiazolinone is a potent preservative with a well-documented risk of allergic contact dermatitis. It is considered safe for use in rinse-off products at regulated low concentrations, but is not recommended for leave-on formulations or for individuals with sensitive skin or a history of allergies.

Research notes

Numerous dermatological studies confirm MCI as a leading contact allergen. Patch test data from the North American Contact Dermatitis Group and European studies show positive reaction rates of 2–5% in tested populations. Regulatory bodies have restricted its use based on this evidence.

Common label clues

Typical concentration: Typically used at 0.0015% to 0.01% in rinse-off products
Regulatory status: Approved for use in rinse-off cosmetics in many countries, including the US and EU, but banned in leave-on products in the EU. Concentration limits vary by region.
Common uses: Shampoos, Conditioners, Body washes, Liquid soaps, Wipes
Environmental note: Isothiazolinones can be toxic to aquatic life at low concentrations. They are not readily biodegradable and may persist in water systems.

Good to know

MCI is almost always paired with methylisothiazolinone (MI) in a 3:1 ratio.
The European Union has banned MCI/MI in leave-on cosmetics since 2017 due to allergy concerns.
In the US, MCI/MI is allowed in rinse-off products but is restricted to a maximum concentration of 0.0015% (15 ppm).

Common questions

What is Methylchloroisothiazolinone in beauty products?

What does Methylchloroisothiazolinone do in a beauty product?

Is Methylchloroisothiazolinone safe for most people?

Who should be careful with Methylchloroisothiazolinone?

Anyone with a history of contact dermatitis or skin allergies People with eczema or very sensitive skin Those using leave-on products (creams, lotions, serums)

Research sources

Cosmetic Ingredient Review (CIR) Safety AssessmentOpen American Contact Dermatitis Society – Allergen of the YearOpen PubMed – Contact allergy to isothiazolinonesOpen European Commission – SCCS Opinion on MI and MCIOpen

Ingredient reviews are educational and are not medical advice. Patch test new products and ask a licensed clinician about persistent irritation, allergies, pregnancy-specific questions, or diagnosed skin conditions.