Methylisothiazolinone

About Methylisothiazolinone

Methylisothiazolinone (MI or MIT) is a synthetic preservative from the isothiazolinone family. It is very effective against bacteria at tiny concentrations, measured in parts per million, which made it popular in shampoos, conditioners, wipes and household products, often blended with its chlorinated cousin methylchloroisothiazolinone.

As its use grew, so did reports of allergic contact dermatitis — itchy, red, sometimes blistering skin where the product touched. Dermatology groups highlighted it as a major allergen in the 2010s, and the European Union went on to ban MI in leave-on cosmetics and set a low limit for rinse-off products. Many brands reformulated as a result, but it still appears in some rinse-off products sold in the U.S.

Risk is highest with products that stay on the skin, such as leave-in conditioners and styling creams, and lower with shampoos that are rinsed off quickly. Once someone becomes allergic, even small exposures can trigger a reaction, so people with a known MI allergy should avoid it in every product.

MI has nothing to do with Curly Girl status, which only concerns cleansers, silicones, alcohols and waxes. If you develop an itchy scalp, hairline or neck rash after a new product, stop using it and see a dermatologist; patch testing can confirm the cause.