Methylisothiazolinone (MI): The Preservative Behind Many Reactions

Methylisothiazolinone (MI) is a common preservative and one of the leading causes of contact allergy in the UK and EU. A plain-English guide to what it is, where it hides, and who tends to react.

What it is

Methylisothiazolinone, usually shortened to MI or MIT, is a preservative. Its job is to stop bacteria, yeast and mould from growing in water-based products, which would otherwise spoil quickly and could become unsafe to use. It belongs to a family of preservatives called isothiazolinones, and it is often used alongside a close relative, methylchloroisothiazolinone (MCI), in a blend sometimes labelled MCI/MI.

MI is effective at very low concentrations and inexpensive, which is exactly why manufacturers adopted it so widely. As concerns grew about older preservatives, MI was taken up as an alternative and spread rapidly through shampoos, wet wipes, liquid soaps and household products through the early 2010s.

Why it is used in skincare and household products

Any product that contains water needs protecting from microbial growth, and MI does that well. It works across a broad range of bugs, stays stable in many formulations, and is cheap, so it found its way into a huge number of everyday washes and wipes. Beyond cosmetics, the same preservative properties made it popular in household cleaners, washing-up liquids, and water-based paints and adhesives.

The trouble is that its very success is what caused the problem. The more products contained MI, the more often people’s skin met it, and the more people became sensitised to it.

The UK and EU rules

MI is one of the clearest recent examples of regulators responding to a real-world surge in allergy. As rates of contact allergy climbed, UK and EU rules were tightened. Since 2015, methylisothiazolinone has not been permitted in leave-on cosmetic products, the category where it caused the most harm because it stays on the skin. That covers items like moisturisers, face creams and sun creams.

It remains permitted in rinse-off products such as shampoos, shower gels and liquid soaps, but only up to a set maximum concentration. The MCI/MI blend is also restricted to rinse-off products within tight limits. Where MI is present in a cosmetic, labelling rules require it to be named in the ingredients list, so it can be spotted and avoided.

These restrictions have helped, but they do not undo existing sensitisation. Someone who became allergic during the peak years still reacts to the smaller amounts now allowed in rinse-off products and to MI in non-cosmetic sources like paints.

Who reacts, and why

MI is one of the leading causes of allergic contact dermatitis identified in patch-test clinics. The reaction is an immune one: after the skin becomes sensitised, later exposures trigger inflammation. It is typically delayed, with itching, redness and sometimes weeping or small blisters appearing a day or two after contact rather than immediately.

People with eczema or a history of contact dermatitis are at higher risk, partly because a compromised skin barrier lets more of the preservative through. But MI allergy can develop in anyone after enough repeated exposure, which is what made the epidemic so striking. Reactions often appear on the hands, face or eyelids, and eyelid dermatitis in particular can be a clue, because thin eyelid skin is very reactive and often catches product transferred by the fingers.

Patch testing by a dermatology clinic is the way to confirm an MI allergy, and both MI and the MCI/MI blend feature in standard test panels.

How to spot it on a label

On an ingredients list look for Methylisothiazolinone, or occasionally the short forms MI or MIT. The related blend appears as Methylchloroisothiazolinone (and) Methylisothiazolinone, sometimes known by the trade name Kathon CG.

Check shampoos and conditioners, wet wipes including baby wipes, liquid hand soaps and body washes, and washing-up liquid and household cleaners. Because MI is also used in water-based paints and glues that are not covered by cosmetic labelling in the same way, someone who is sensitised may need to think beyond toiletries when tracking down what is causing a flare.

A balanced, honest take

It is fair to be direct about MI: unlike ingredients that are largely fine and only trouble a small minority, MI genuinely drove a measurable rise in contact allergy, and that is why the rules were tightened. At the same time, this is not a reason to panic about every rinse-off product. The restrictions since 2015 have reduced new cases, and for people who are not sensitised, MI in a shampoo that is on and off the skin in seconds is a low-risk exposure.

The honest summary is that MI is a real and common allergen with a documented history, worth avoiding if you have reacted to it or have eczema-prone skin, but not something the whole population needs to fear. The regulatory response here worked broadly as intended.

What to do if you’re avoiding it

If you know or suspect MI is a trigger, the practical steps are straightforward. Read the ingredients on your washes, wipes and cleaners and switch to versions preserved with something else. Fragrance-free ranges aimed at sensitive skin are less likely to use MI, but always check the actual list rather than trusting front-of-pack claims. Remember non-cosmetic sources: choose low-biocide or preservative-free wipes and be cautious in freshly painted rooms if you have reacted before.

If you have a persistent, unexplained rash, especially on the hands, face or eyelids, see your GP or pharmacist, and ask about a referral for patch testing so any allergy can be confirmed properly. This guide helps you read labels; it is not medical advice.

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.)

Written and reviewed by the Sensitive Skin Lab editorial team.