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.
In short
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)
How it's used
In supplements
Not an oral ingredient.
In skincare
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.
Popular products containing Methylisothiazolinone
- Some shampoos and liquid soaps (rinse-off) — Low-level MI preservationVarious · Skincare
- Household cleaners and paints — The industrial-scale usesVarious · Skincare
- Legacy leave-on products (pre-2017, EU) — Now reformulated or withdrawnVarious · Skincare
Product names and brands are shown for reference and belong to their respective owners.
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. https://pubmed.ncbi.nlm.nih.gov/27743480/
- EU restriction of methylisothiazolinone. European Commission. https://ec.europa.eu/growth/sectors/cosmetics/legislation_en
- Preservatives and contact dermatitis. PubMed. https://pubmed.ncbi.nlm.nih.gov/29264868/