Reviewed November 11, 2026

Methylisothiazolinone: The Preservative the EU Banned From Leave-On Products, and Where It Still Hides

Methylisothiazolinone, MI on the INCI list, is a preservative that kills microbes so effectively it also trains skin to react to it. The EU banned it from leave-on cosmetics in February 2017 and capped it at 15 parts per million in rinse-off products, after patch-test clinics found 13.4 percent of tested patients reacting to it. The US still permits it. If your skin flares and a leave-on product lists MI, you have your first suspect, and the INCI list is the only place that will tell you.

The name to search for on the INCI list

MI hides behind three names. Methylisothiazolinone is the full INCI name. Methylchloroisothiazolinone, MCI, is its chlorinated cousin, and the two are sold together as a 3 to 1 blend under the trade name Kathon CG, a product Dow developed in the late 1960s. On a label you will see one of those words, usually near the end of the list, because preservatives are used in tiny amounts. That position is the trap: the end of the INCI list is where most people stop reading, and for a sensitizer, the dose does not have to be large to matter.

The blend has been in cosmetics since the 1980s. MI on its own is newer: formulators started using it solo in 2005, at concentrations up to 100 parts per million, because it is a strong biocide at very low doses. Within a few years, dermatology clinics across Europe were seeing a sharp rise in allergic contact dermatitis traced to it. The American Contact Dermatitis Society named MI its Allergen of the Year in 2013, an award that is really a warning label for the whole field.

What the EU did, and the numbers behind it

The EU Scientific Committee on Consumer Safety looked at the data in 2015 and wrote, in unusually blunt language, that 100 parts per million of MI in cosmetics is not safe for the consumer. For leave-on products, including wet wipes, the committee found no safe concentration at all. Regulation (EU) 2017/1224 followed: MI was banned from leave-on cosmetics from February 2017, and the rinse-off limit was cut from 100 parts per million to 15, a sevenfold reduction. The MI and MCI blend sits under a separate entry at the same 15 parts per million ceiling, rinse-off only.

The patch-test numbers explain why the committee moved. In the North American Contact Dermatitis Group 2015 to 2016 cycle, 13.4 percent of 5,597 patch-tested patients reacted to MI, and it carried the highest significance-prevalence index in the whole panel, meaning the reactions were not incidental findings but the reason those patients were in the clinic. A 2026 meta-analysis of more than 1.5 million dermatitis patients put contact allergy to MI at about 5.5 percent of those tested. Those are not niche numbers. They are the reason MI went from workhorse preservative to restricted substance in under four years.

Why the US still allows it

The US Cosmetic Ingredient Review panel reached a different conclusion: MI is safe in rinse-off products up to 100 parts per million, and safe in leave-on products when formulated to be non-sensitizing, a judgment left to a quantitative risk assessment by the manufacturer. The FDA sets no specific concentration limit. So a moisturizer sold in the US can legally contain MI in a leave-on formula that the EU would refuse to place on the market, and a wipe that caused the original epidemic can still be sold here if the manufacturer judges its formula non-sensitizing.

This is not a case of one regulator being right and the other wrong. It is a case of two different risk tolerances for the same data, and the practical consequence lands on the person with reactive skin. The EU rule protects you by default. The US rule asks you to protect yourself by reading the list. That is the whole argument for checking the INCI list rather than trusting the front of the pack.

Where it still shows up

Start with wipes, the product class that drove the epidemic. A wipe is a leave-on product by definition: whatever is on the cloth stays on the skin. If a wipe lists MI or MCI, put it back. Then check rinse-off products, where MI is still legal in the EU at 15 parts per million and in the US at 100. A shampoo or body wash at those levels is a brief exposure, and most people tolerate it, but if you are already sensitized, even a rinse-off exposure can elicit a reaction, and the SCCS was explicit that it had no data on elicitation at 15 parts per million.

The last place to look is outside cosmetics entirely. MI is used in paints, glues, cleaning products, and industrial fluids, and a 2017 case report traced a reaction to the glue in a shoe. A skin reaction that follows no new cosmetic is worth checking against household exposures, and a dermatologist can patch test MI at 2,000 parts per million in water to confirm the allergy. If the test is positive, the fix is not a better moisturizer. It is a list of names to avoid, starting with the two above.

Frequently asked questions

Is methylisothiazolinone still legal in the US?

Yes. The FDA sets no specific concentration limit for MI, and the US Cosmetic Ingredient Review panel considers it safe in rinse-off products up to 100 parts per million and in leave-on products when formulated to be non-sensitizing. That is far looser than the EU, which banned MI from leave-on cosmetics in 2017 and capped rinse-off products at 15 parts per million.

What does MI look like on an ingredient list?

Look for methylisothiazolinone or methylchloroisothiazolinone, usually near the end of the INCI list. The two are also sold as a 3 to 1 blend under the trade name Kathon CG. Any of those names means the product contains an isothiazolinone preservative, and for sensitized skin that is the line to check first.

I reacted to a wipe years ago. Could MI be why, and does the allergy fade?

It is a strong candidate. Wipes were the product class behind the MI allergy epidemic, because a wipe leaves its preservative on the skin. Contact allergy to MI does not reliably fade on its own; once sensitized, most people stay sensitized, which is why a dermatologist patch test at 2,000 parts per million in water is worth doing before you write the reaction off as a one-off.

If a product says hypoallergenic, can it still contain MI?

Yes. Hypoallergenic has no legal definition in US cosmetics, so it is a marketing claim, not a formula standard. A product can carry the word on the front and MI on the back. Only the INCI list tells you what is actually in the formula.