Methylisothiazolinone: The Preservative Behind an Allergy Epidemic
INCI: Methylisothiazolinone · also listed as MI, MIT, MCI/MI, Kathon CGApproved on its own in Europe in 2005; an allergy epidemic by 2013; banned from EU leave-on products in 2017. Still legal at higher levels in the US.
Effective, legal in shampoo, and worth avoiding
MI preserves a product well and does nothing for hair. Its problem: it sensitises skin at extraordinarily low doses. Across 14 European clinics, 6% of everyone patch-tested in 2015 reacted to it. On a scalp the reaction looks like dandruff or eczema at the hairline, ears and neck.
The EU banned it from leave-on cosmetics in 2017 and cut rinse-off products to 0.0015% in 2018; the US has no limit. Phenoxyethanol and sodium benzoate preserve a shampoo just as well, so there is no benefit to weigh against the risk. Skip it, and its partner MCI/MI too.
- INCI name
- Methylisothiazolinone
- Also on labels
- MI, MIT. Often paired as methylchloroisothiazolinone / methylisothiazolinone (MCI/MI, trade name Kathon CG), more potent still.
- Job in hair care
- Broad-spectrum preservative. Keeps a water-based formula free of bacteria and mould in a warm bathroom.
- Found in
- Older and value-range shampoos, conditioners and wet wipes. Rare in new EU launches, common in US mass-market lines.
- Typical level
- EU rinse-off: max 0.0015% (15 ppm). US rinse-off: up to 0.01% (100 ppm) under the industry panel's guidance.
- Regulatory status
- Banned in EU leave-on products since 12 February 2017; rinse-off capped at 0.0015% from 2018. The UK mirrors the EU. No US federal limit.
- Hairmony class
- ActiveScored by the Hairmony engine, so it actually moves which products you get recommended.
- Scored for
- −scalp sensitivity−scalp healthCarried purely as a contraindication: the engine's heaviest sensitive-scalp penalty plus a scalp-health penalty, with no hair benefit to offset either.
What it does on your scalp
Nothing for your hair. A preservative protects the bottle, and MI does that well. On the scalp it is a potent contact allergen: it binds to skin proteins and primes the immune system, so a later exposure to a far smaller amount triggers dermatitis.
The scalp itself is thick and resistant, so the reaction shows up where product runs and lingers: an itchy, scaly hairline, red ears, a rash on the neck or eyelids. It gets treated as dandruff or "sensitive skin" for months before anyone reads the label.
Worth tolerating if
- The product is rinse-off, EU-compliant, and you have never reacted to it
- There is no alternative on your shelf tonight
Avoid if
- You have eczema, a sensitive scalp or any past cosmetic reaction
- The product is a leave-in, cream, serum or wipe
- Your hairline or ears itch and dandruff shampoo has not helped
Where it hides on a label
Why the EU banned it from leave-on products
Until 2005, MI was allowed only in a fixed mix with MCI, at 15 ppm. Then the EU approved it alone at 100 ppm, and it spread into moisturisers, wipes and leave-in hair products. Patch-test positives climbed until the American Contact Dermatitis Society named it Allergen of the Year in 2013.
The leave-on ban took effect in February 2017; rinse-off products were cut to 15 ppm the next year. Across Europe, positive patch tests halved from 6% to 3% within two years. New cases are falling, but the already sensitised stay sensitised, which is why it is still worth reading for.
In practice: if your hairline itches and flakes, read the last three ingredients of everything you use before buying another dandruff shampoo. Swap anything listing MI or MCI/MI.
What we based this on
- Regulation. Regulation (EU) 2016/1198, banning MI in leave-on products from 12 February 2017; Regulation (EU) 2017/1224, capping rinse-off products at 0.0015% from 2018.
- The epidemic. Uter et al., JEADV 2020: 14 European centres, positives down from 5.97% (2015) to 2.96% (2017). Havmose et al., Contact Dermatitis 2021: leave-on sources fell from 24.8% to 6.2%.
Not medical advice. This page describes how an ingredient behaves on hair and scalp. If you have a persistent rash, sores, sudden shedding or a diagnosed scalp condition, see a dermatologist before changing anything.
Frequently asked
Is methylisothiazolinone banned?
Partly. The EU banned it from all leave-on cosmetics in February 2017 and capped rinse-off products such as shampoo at 0.0015% (15 ppm) from 2018; the UK follows the same rules. The US has no federal limit, and its industry review panel allows up to 100 ppm in rinse-off products.
What does a methylisothiazolinone allergy look like on the scalp?
Usually not like a rash on the scalp itself. It shows up where product runs and lingers: an itchy, flaky hairline, red or weeping skin behind the ears, a rash on the neck or eyelids. It is often mistaken for dandruff, and it does not respond to anti-dandruff shampoo.
Is methylisothiazolinone safe in shampoo?
At the EU limit of 15 ppm, the risk of becoming newly sensitised from a rinse-off product is low. If you are already allergic, that amount is enough to trigger a reaction. Since other preservatives do the same job without the allergy record, there is little reason to accept it.
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