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Methylisothiazolinone

Methylisothiazolinone in skincare: Preservative. What it does on skin, how strong the evidence behind the claim actually is, the concentrations in normal use, who reacts to it, and where it stands in pregnancy and sunlight. On a label it appears as Methylisothiazolinone, Methylchloroisothiazolinone.

Preservatives, surfactants and fragrancePreservativeEvidence: Strong

On the label: Methylisothiazolinone, Methylchloroisothiazolinone. Also called: MI, MCI/MI, the "Kathon" preservative system.

This is an ingredient reference, not a diagnosis and not a treatment plan. It describes what substances are and what the published evidence supports; it does not tell you what your skin has or what to put on it. A rash that spreads, hurts, weeps, does not settle, or comes with feeling unwell is a reason to see a doctor or pharmacist, not to change serum.

What it is

A very potent broad-spectrum preservative active at parts-per-million concentrations, used alone or as a mixture with its chlorinated relative. It is the ingredient at the centre of the most instructive story in modern cosmetic safety, and it is the reason a dermatologist testing you for a rash will ask about wet wipes, shampoo and the paint in your hallway.

What it does on skin

It kills bacteria and fungi extremely efficiently in water-based products at concentrations too low to affect the feel or smell of the formula. That efficiency is exactly why it spread so fast when formulators needed a paraben replacement, and it is also why so little of it is needed to sensitise someone.

What the evidence is worth

Evidence: Strong

Its preservative performance is not disputed. What is documented beyond dispute is the consequence of using it in leave-on products: European contact-allergy clinics recorded a steep, sustained rise in positive patch tests through the late 2000s and early 2010s, described in the dermatology literature as an epidemic of contact allergy, and regulators acted on that evidence.

How it is used

Nothing for a consumer to optimise. What matters is recognising the name on a rinse-off label if you have ever had an unexplained hand or face dermatitis, and asking to be patch tested rather than working through products by trial and error.

Typical concentration in normal use: Parts per million. That is a description of the market, not a recommendation, and it is not a dose.

Who reacts to it

Allergic contact dermatitis, sometimes severe, and once you are sensitised it is for life. It can also present as airborne dermatitis — a facial and eyelid rash with no product applied there — because isothiazolinones are used as preservatives in water-based wall paints, and freshly painted rooms have caused exactly that.

Pregnancy and breastfeeding

Generally considered fine

Not a pregnancy issue. It is an allergy issue, and it behaves the same way in pregnancy as at any other time.

Sunlight

No change

No effect on light sensitivity.

Rules and labelling

The EU banned methylisothiazolinone in leave-on cosmetic products and limited it in rinse-off products to 15 parts per million, having earlier restricted the chlorinated mixture. This is the clearest case in cosmetics of a regulator responding to clinical allergy data rather than to laboratory toxicology.

What it goes with, and what it does not

Not applicable. If you are sensitised, the practical work is avoidance across cosmetics, household cleaners, wet wipes and paint, and a patch-test clinic will give you the list.

One thing people get wrong

The myth is that "free-from" reformulation always makes a product safer. This is the case that disproves it: removing a low-allergy preservative created a market for a high-potency one, its use rose sharply in leave-on products, allergy rates followed, and it took a regulatory ban to reverse. The preservative that was replaced was the one causing fewer reactions.

Sources: EU Cosmetics Regulation (EC) No 1223/2009 and its annexes, as published in the European Commission's CosIng database, for every statement about what is permitted, restricted or banned · opinions of the Scientific Committee on Consumer Safety (SCCS) and safety assessments of the Cosmetic Ingredient Review (CIR) for the concentration positions · standard dermatology reference material for mechanism, tolerability and contact-allergy patterns · checked 2026-09-06. Deep links are deliberately omitted where a stable public URL could not be confirmed.

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