What is Methylchloroisothiazolinone?

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  • Written By: Mary McMahon
  • Edited By: Kristen Osborne
  • Last Modified Date: 29 August 2019
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Methylchloroisothiazolinone is a chemical compound with preservative properties used in a wide variety of products from cosmetics to paints. This ingredient can be a skin irritant and allergen in high concentrations and is typically used in dilute forms when it is employed in products intended for direct skin contact. People who have a history of reacting to it should read ingredient labels carefully for disclosures of methylchloroisothiazolinone and may want to discuss the risks with a dermatologist or allergy specialist.

Use of this product dates to the 1970s. It is both antibacterial and antifungal, inhibiting growth of organisms that might cause spoilage. When used in cosmetics, the goal is to keep the cosmetic clean, reducing the risk of developing an infection or reaction after using the cosmetic. In other products, the chemical is added to resist fungal or bacterial growth, allowing people to rely on paints, stains, and other products without worrying about the development of discolorations and other problems.

Research on methylchloroisothiazolinone has shown that in pure form, it can be extremely irritating to the skin, causing itching, redness, and swelling. Diluted preparations are sold commercially for use in skin care products and other things that will be used on the skin. Most patients should not react to the dilute compound in these products, although people with extremely sensitive skin may develop contact dermatitis.


If people notice itching, redness, and other signs of irritation after applying a product to the skin, they should wash the skin off and discontinue use of the product. It is advisable to consult a doctor to get information about likely allergens. People can also compare and contrast ingredient lists to see which ingredients tend to come up in products they react to. Sometimes, a variety of chemicals could be the culprit and some detective work may be required to find out why someone experiences skin reactions.

People with a known history of reactions to methylchloroisothiazolinone should make sure their pharmacists are aware of this. When medications are prescribed for topical use, the pharmacist can catch prescriptions for drugs with the irritant in them, and in cases where pharmacists compound medications specially for patients, they can select a different preservative to avoid triggering a reaction. This information should also be recorded in patient charts so doctors know to avoid prescriptions for medications known to contain the chemical and to prevent situations where topical applications with methylchloroisothiazolinone are applied to patients in hospital environments.


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Post 3

Have just had a bad reaction to tresemme shampoo. Won't buy it again.

Post 2

@JaneAir - I have a friend with this allergy. She actually discovered it when she had a bad reaction to some medication. Now she has to make sure to pay really close attention to what makeup she purchases.

She's had some good success with some more natural brands of makeup though. Most natural brands have less preservative and chemicals in them so they're less likely to contain methylchloroisothiazolinone.

Post 1

While I'm glad to know that my makeup has an antibacterial in it, I'm a little disturbed it's also a skin irritant!

I think I'm going to share this with a friend of mine. She broke out pretty badly after trying some new makeup recently. Maybe she's sensitive to methylchloroisothiazolinone! I'm going to see if she can compare her existing makeup to the one that made her break out and see what the difference in ingredients is.

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