What is methylisothiazolinone? What EU, US and journal sources say

What it is

We work from documents; we have not tested any products. Methylisothiazolinone, usually shortened to MI, appears on ingredient lists as a preservative. PubChem, the US National Library of Medicine’s compound database (retrieved 2026-11-14), describes it as “a powerful biocide and preservative” and “the minor active ingredient in the commercial product Kathon.” It gives roles as an antimicrobial agent, an antifungal agent and an antifouling biocide, and lists CAS number 2682-20-4. Synonyms include 2-Methyl-4-isothiazolin-3-one and Neolone 950.

A preservative is there to keep a product from spoiling. Preservatives are not inherently harmful, and this page does not argue otherwise. It reports what named sources say about this one.

What the EU regulation text says

The text of Commission Regulation (EU) 2016/1198, from the EU Publications Office (retrieved 2026-11-14), says MI “is authorised as a preservative in cosmetic products at concentrations up to 0,01 % weight/weight (100 ppm) through entry 57 of Annex V to Regulation (EC) No 1223/2009.”

It then records the reasoning for changing that. The Scientific Committee on Consumer Safety (SCCS) adopted an opinion on MI (sensitisation only) on 12 December 2013. The regulation says the SCCS “concluded that current clinical data indicate that the concentration of 100 ppm of methylisothiazolinone in cosmetic products is not safe for the consumer,” and that for leave-on products, including wet wipes, “no safe concentrations of methylisothiazolinone for induction of contact allergy or elicitation have been adequately demonstrated.”

The text goes on to say that, because of the increased incidence of allergies induced by MI, “this substance should therefore be banned in leave-on products.” A footnote adds that MI is also regulated, in a mixture with methylchloroisothiazolinone, in entry 39 of Annex V, and that the two entries are mutually exclusive in the same product.

The excerpts we hold are the opening statements of the regulation. We did not read its operative articles or application dates, so we say nothing here about what applies to rinse-off products now, or when any change took effect. For current law, read the regulation itself.

What PubChem adds on safety

PubChem also quotes a conclusion by the Expert Panel for Cosmetic Ingredient Safety: that MI “is safe for use in rinse-off cosmetic products at concentrations up to 100 ppm (i.e. 0.01%) and safe in leave-on cosmetic products when they are formulated to be non-sensitizing, which may be determined based on a QRA or similar methodology.” That is the panel’s statement as quoted by PubChem, and we do not weigh it against the EU text. The two statements come from different bodies and are different in scope. We found no source in our files that sets out the US legal position, so we do not state one.

What journal abstracts report about contact allergy

We read four PubMed abstracts on MI contact allergy. Each is a report by its authors, and we give the details as reported.

• PMID 21777214, British Journal of Dermatology 2011. One review reports that MI was permitted in cosmetics in 2005, that the prevalence of MI contact allergy was “already around 1.5%,” and that many MI-allergic patients reacted to 50 ppm, half the maximum permitted concentration. It suggests the cases could be “the first sign of an epidemic.”
• PMID 24117738, Contact Dermatitis 2013. One study of patch-tested patients at Gentofte Hospital reports MI allergy rising from 2.0% to 3.7% between 2010 and 2012, and says cosmetics were the most common relevant exposure. It also reports MI in 3.3% of cosmetics on the Danish retail market.
• PMID 23340392, Dermatitis 2013. One report names MI the American Contact Dermatitis Society Contact Allergen of the Year for 2013. It says the MCI/MI patch test mix misses approximately 40% of allergy to MI, and that the frequency of allergy in the United States “is unknown.”
• PMID 26412037, European Journal of Dermatology 2015. One report of two multicentre studies (8,680 patients in Belgium and 7,874 in France) gives a sensitization rate of about 6.0% in 2012 and 7.0% in 2013. It says the data confirm sensitization is primarily caused by cosmetics, household detergents and water-based paint.

These are patch-tested patient groups at particular clinics and are not general-population rates. Also, the abstracts are from 2011 to 2015, so they describe the period around the EU action and not today. We did not find or read systematic reviews or trials in our files.

On the label

The official EU glossary of common ingredient names (Commission Decision (EU) 2019/701), retrieved 2026-11-14, contains an exact entry for each of these names:

Glossary name	Entry number
METHYLISOTHIAZOLINONE	14597
METHYLCHLOROISOTHIAZOLINONE	14551
BENZISOTHIAZOLINONE	2906

All three names we looked up had an exact entry; none was missing.

Where it sits on this site

This part is our rule, labelled as ours. Methylisothiazolinone is not fragrance, parfum or aroma, and no source we hold calls it one. It does not fail a fragrance-free check on that ground. Our hypoallergenic-lens review keeps a watch list of preservatives, and MI is on it. That is a site rule and not a regulator’s finding, and we do not claim a product with or without MI suits anyone.

For the fragrance terms we do check, see what is parfum and how to read an ingredient list for parfum and aroma. Other preservative explainers: what is phenoxyethanol, parabens in skincare and formaldehyde releasers in skincare. For the fragrance side of allergy labelling, see the EU 26 fragrance allergens.

What this page cannot tell you

It cannot say whether MI is a problem for you. For questions about your own reactions, or a child’s, ask a clinician, or look to an allergy or dermatology society.