Methylisothiazolinone
Also known as: MI, MIT, 2-Methyl-4-isothiazolin-3-one
The preservative that caused a real allergy epidemic — effective at tiny doses, infamous for leave-on sensitivity.
Methylisothiazolinone (MI) is a powerful preservative effective against bacteria and fungi at concentrations far lower than most alternatives — which is exactly why the industry embraced it in the 2000s as a paraben replacement. The embrace went too far: MI's use in leave-on products and at rising concentrations triggered a genuine epidemic of allergic contact dermatitis, peaking in the 2010s as the second-most-common cosmetic allergen in patch-test clinics, at rates reaching 10% in some populations. Regulators responded by banning MI in EU leave-on products (2017) and capping rinse-off concentrations — a rare case of an ingredient's allergy problem driving regulatory action. In current rinse-off formulations at low levels, risk is substantially reduced but not eliminated.
- Potent preservative effective at ppm concentrations
- 2010s: a real contact-allergy epidemic (up to 10% patch rates)
- EU banned MI in leave-on cosmetics (2017)
- Rinse-off use at low levels remains permitted
What is Methylisothiazolinone?
Methylisothiazolinone is the preservative that worked too well — and the story matters because it is one of the few times the ingredient-allergy system actually reshaped regulation. Potent at concentrations a fraction of its alternatives, MI became the industry's paraben replacement of choice, only for patch-test clinics to document an epidemic of allergic reactions as exposure climbed. The 2017 EU ban in leave-on products and tightened rinse-off limits followed, and allergy rates have since stabilized. MI remains a legitimate rinse-off preservative — but it is the ingredient that taught the industry the cost of over-using a sensitizer.
Where it comes from
Natural sources
- Fully synthetic isothiazolinone compound
- Introduced as a preservative in the 1970s, widely adopted 2000s
How it's made
Used at 50–100 ppm in rinse-off products; EU prohibits leave-on use; often paired with methylchloroisothiazolinone (Kathon CG).
What it does in products
Benefits
Powerful preservation at low doses
strong evidenceEffective at concentrations far below most alternatives — the original reason for its adoption in water-based formulas.
Paraben-free compatibility
strong evidenceFunctioned as the workhorse of early paraben-free systems before allergy data prompted restrictions.
Regulated risk management
strong evidenceCurrent rinse-off limits and leave-on bans have substantially reduced exposure and stabilized allergy rates.
Side effects & safety
Safety considerations
- Rinse-off use at regulated levels is the standard; leave-on avoidance is prudent globally.
- None.
- Not for: Patch-test-confirmed MI allergy
- Not for: Leave-on product use (EU-banned; prudent everywhere)
Amounts studied & product use
These figures describe research or common product use, not a personal dose or treatment recommendation.
In supplement studies and products
Not an oral ingredient.
In skincare studies and products
50–100 ppm in rinse-off products; leave-on use prohibited in the EU.
Regulatory status
- Permitted cosmetic preservative in the US; EU bans leave-on use and caps rinse-off at 15 ppm (mixtures).
- INCI name on cosmetic labels: Methylisothiazolinone.
- CIR reviewed; EU restriction reflects the allergy epidemic.
How the epidemic happened
As parabens fell from favor, MI use expanded from industrial products to rinse-off and then leave-on cosmetics, with concentrations rising. Because MI is a potent sensitizer, cumulative exposure crossed allergy thresholds in a meaningful slice of the population — patch-test rates hit 6–10% in European clinics by 2013. The sequence is a textbook case of sensitization dynamics at population scale.
The regulatory response
The EU banned MI in leave-on products (2017) and capped rinse-off concentrations; the US has permitted continued use with lower industry concentrations. Allergy rates have since declined from their peak. The episode demonstrates that preservative safety is a dose-and-exposure equation, not a static property.
Frequently asked questions
Should I avoid methylisothiazolinone entirely?
If you have confirmed MI allergy, yes. Otherwise, avoid it in leave-on products (EU-banned, prudent everywhere); rinse-off use at current low levels is substantially lower-risk.
Why did MI become so common?
It is effective at tiny concentrations, compatible with 'paraben-free' marketing, and cheap — until the allergy data changed the math.
Is MI the same as Kathon CG?
Kathon CG is a mixture of MI and methylchloroisothiazolinone. MI is the more common sensitizer, and EU limits cover the combination.
The bottom line
Methylisothiazolinone is the preservative that proved sensitization is a population-level phenomenon — now appropriately restricted, and a case study in ingredient vigilance.
References
- MI contact allergy epidemic: clinical data. PubMed Central
- EU restriction of methylisothiazolinone. European Commission
- Preservatives and contact dermatitis. PubMed