Ingredient guide

Methylisothiazolinone (MI): The Preservative Behind Many Reactions

Methylisothiazolinone (MI) is a common preservative and a leading cause of contact allergy. Here's what it is, where it hides, and who tends to react.

What it is

Methylisothiazolinone (MI, sometimes MIT) is a preservative used to stop bacteria and mould growing in water-based products. It is effective and cheap, which is why it spread quickly through rinse-off and wet products in the 2010s.

Why it became a problem

That rapid spread came with a sharp rise in contact allergy. Dermatology groups across Europe described it as an epidemic of MI allergy, and concentrations in leave-on products have since been restricted. It remains permitted in many rinse-off products, so it is still worth knowing.

Who tends to react

People with eczema or a history of contact dermatitis are most at risk, but MI allergy can develop in anyone after repeated exposure. Reactions are typically delayed: itching, redness, sometimes weeping, appearing a day or two after contact, often on the hands, face or eyelids.

Where it hides

Check shampoos, wet wipes (including baby wipes), liquid hand soaps, washing-up liquid and some household cleaners. On labels look for Methylisothiazolinone, MI, MIT, or the related blend Methylchloroisothiazolinone (MCI).

Check products against your list

To check a product, SkinSAFE lets you filter a product catalogue to screen out methylisothiazolinone and other allergens, and a free analyser like Skincarisma lets you paste a product at your desk and see methylisothiazolinone flagged in the ingredient list. These rate a product on general criteria rather than against your own list.

Once you know what you are screening for, a personal-list app like ClearaScan lets you save methylisothiazolinone once and scan any product to flag only your triggers. It also keeps a Reaction Journal for flare-ups, a shared Care Circle so family or carers can scan for you, and a Trusted Products list for items you have cleared, and it is currently in early access. (Disclosure: our editor co-founded ClearaScan, and we are not paid to mention the others.)

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A note on this content. The Sensitive Skin Lab publishes general educational information, not medical advice. If you suspect you have an allergy or sensitivity, consult a qualified dermatologist or allergist. Product formulations and labels change without notice, so always check the ingredients on the product itself.