What Is Methylisothiazolinone and Why Is It In My Shampoo?

If you're like us and constantly reading product labels, your eyes might have lazily glazed over an ingredient called methylisothiazolinone in some of your hair care products. The additive, which we'll call MIT for short going forward, is used to preserve haircare products, preventing them from growing mold and bacteria. But is it actually doing more harm than good? We spoke with board-certified dermatologist Geeta Yadav, MD, as well as cosmetic chemist Kelly Dobos to get all the info on MIT, including any potential negatives. Read on to learn more.

What Is Methylisothiazolinone?

Methylisothiazolinone (MIT) is a "chemical substance that is effective in eliminating and controlling the growth of potentially harmful bacteria," explains Dobos.

Benefits of Methylisothiazolinone for Hair

MIT does not have any benefits specifically for your hair. It won't make it shiny, more manageable, thicker, or stronger. It is strictly used in cosmetics as a preservative and bacteria-inhibitor, which is obviously an important thing we all want in a product we're keeping in a warm, damp place like the bathroom.

As Dobos explains, "Consumer products like shampoo and conditioner need to have a long shelf life, from production in a manufacturing plant to transportation and time on store shelves and then throughout consumer use. The conditions the product is subjected to, like storage in a warm, damp environment like a shower stall, are all things that need to be considered—and preservatives like MIT keep our hair care products safe for use for longer."

Even though MIT doesn't have direct benefits for hair, it does have a number of benefits in a product formulation. Dobos adds, "MIT protects products at a very low-use level. It's also water soluble, making it very easy to mix in, It's also stable over a broad pH range, making it suitable for many types of formulas like shampoos and conditioners."

Hair Type Considerations

MIT can be used by all hair types, according to Yadav. It is more important to be conscious of any allergies or skin you might have before using products containing the ingredient. (More on this, ahead.)

Potential Side Effects

Yadav says MIT is generally regarded as a safe ingredient. "Keep in mind that liquid products stored in damp areas are prone to bacteria growth—it's better to have those formulas be well preserved rather than risk rubbing bacteria and fungus into your scalp. However, methylisothiazolinone has been shown to be irritating to some. If you are prone to skin sensitivity, sensitized skin, or allergies, you may consider steering clear of methylisothiazolinone."

Some symptoms of an allergic reaction to MIT include an itchy, scaly rash, and possibly blistered skin. If it gets worse, it can resemble other skin conditions like eczema.

If you're particularly concerned about MIT in your hair care, there are a number of brands that specifically do not include it in their formulations, including Acure Organics, Andalou Naturals, and SheaMoisture.

How to Use Methylisothiazolinone for Hair

As mentioned, MIT is a preservative used in haircare products like shampoos and conditioners. Because MIT can cause allergic reactions in a small percentage of the population, Yadav suggests being mindful of the amount of the ingredient is in the product, especially if you have any skin conditions. "If you have a known sensitivity be sure to read ingredient statements on cosmetics and personal care products, because of the small percentages used preservatives are typically at the end of the list."

FYI, this doesn't mean that if you do happen to find MIT in your haircare, that you need to toss your products. If you haven't had any skin reaction, sensitivity, or irritation, you can safely continue to use your shampoos and conditioners.