Methylisothiazolinone on an ingredient list: what DermNet says about this preservative allergy

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Reviewed October 17, 2026

The short answer

Methylisothiazolinone, or MI, is a preservative that keeps products from spoiling. DermNet calls it a strong allergen and describes an epidemic of allergic dermatitis once it moved into cosmetics and household goods around 2005. The ACDS, a US contact dermatitis society, picked it as its 2013 allergen of the year.

What is it?

DermNet says this preservative was first used in paints, glues and cleaners, often mixed with methylchloroisothiazolinone (MCI). After years of home use it proved a strong sensitizer.

Where DermNet says it turns up

Its list includes moisturizers, creams, shampoos, conditioners, soaps, body washes, bubble bath, sunscreens, foundations, mascara, makeup removers, baby wipes and moist wipes, plus detergents, paints and glues. DermNet names two main exposure routes: wipe-type products put straight on skin, and shampoos or liquid soaps, which are rinsed off but can still cause dermatitis. MI is volatile, so freshly painted rooms can also trigger airborne reactions.

What DermNet says about the allergy

• It calls the allergy extremely common, with up to 20 to 25 percent of patch-tested patients across Australia and New Zealand reacting during 2015 to 2016.
• DermNet reports that in July 2015 a European safety committee barred MI from new leave-on goods and set a rinse-off limit so low that MI stops working as a preservative.
• At the start, only some of the exposed skin may react, such as hands, face, eyelids or scalp. Very sensitive people can develop dermatitis over the whole body.
• Diagnosis uses history and patch testing. DermNet cites a 2015 study in which 27 percent of the people allergic to MI would have gone undetected by testing only the MCI/MI mix.

Avoidance and testing

• DermNet says to read labels, since MI is usually near the end of an ingredient list, and not to use unlabelled products. Products from one brand can use different preservatives.
• DermNet says every isothiazolinone chemical may trigger the same allergy, and that other preservatives can cause allergy too.
• Testing a leave-on product at home is possible, but only after talking with a dermatologist: dab a little on the bend of the elbow two times daily for three weeks. Washes such as shampoo, soap and cleanser are not suited to home testing.
• DermNet says treatment is with topical corticosteroids and emollients, and severe cases may need oral corticosteroids.

What this page leaves out

This page rests on one DermNet topic page, supported by DermNet's general preservative overview, which lists isothiazolinones among preservative classes that can cause dermatitis in susceptible people. It does not say how much MI any current US product contains, because the source gives no US limit.

How to read it on a label

US cosmetic labels generally list ingredients from the largest amount down, but anything at 1% or less can appear in any order after those above 1%. Its position does not tell you the exact concentration. Judge the whole product, how it feels on your skin, and what changes when you add or drop it.

What not to conclude

This page does not say a product with MI will cause a reaction, or that a product without it is free of preservative allergy risk. If a product irritates your skin, stop using it and ask a dermatologist.