Irritants , Preservatives

Preservative that’s generally recommended for use only in rinse-off products such as cleansers or shampoos. Although used in leave-on products, methylisothiazolinone is known to be sensitizing when used in leave-on products. In fact, the American Contact Dermatitis Society named methylisothiazolinone its Allergen of the Year in 2013! [1]

As with most ingredients with problematic potential, much depends on the amount used, but, as a general rule, if you see methylisothiazolinone in the middle of an ingredient list for a leave-on product, you should avoid it, particularly if you have sensitive skin or if this preservative is used in conjunction with sunscreen actives like octinoxate or avobenzone. Methylisothiazolinone is active against bacteria, but has weak antifungal properties. Its use in both rinse-off and leave-on cosmetics products is restricted to low amounts as a means to avoid irritation while maintaining its efficacy as a preservative, but even in the low amount permitted (0.01%) has raised questions about it being more sensitizing than most other preservatives. [2]

In combination with methylchloroisothiazolinone, this preservative becomes a more frequent allergen in leave-on products, particularly for hair care, baby care, and feminine hygiene. [3,4] To be clear: Neither of these preservatives is considered problematic when used in rinse-off products such as cleansers or body washes; it’s the leave-on products, especially those meant for use around the eyes.

References Cited:

  1. Scherrer M, Rocha V. Increasing trend of sensitization to Methylchloroisothiazolinone/methylisothiazolinone (MCI/MI). An Bras Dermatol. 2014;89(3):527.
  2. Flower C, Meredith E. Preserving with Methylisothiazolinone. Cosm & Toil. 2014;129(2):24.
  3. Geier J, Lessmann H, Schnuch A, Uter W. Recent increase in allergic reactions to methylchloroisothiazolinone/methylisothiazolinone: is methylisothiazolinone the culprit? Contact Dermatitis. 2012;67(6):334-41.
  4. Travassos A, Claes L, Boey L, Drieghe J, Goossens A. Non-fragrance allergens in specific cosmetic products. Contact Dermatitis. 2011;65(5):276-285.
See preservatives , methylchloroisothiazolinone

«Back to Ingredient Dictionary

About the Experts

Paula Begoun is the best-selling author of 20 books on skin care and makeup. She is known worldwide as the Cosmetics Cop and creator of Paula's Choice. Paula's expertise has led to hundreds of appearances on national and international television including:

View Media Highlights

The Paula's Choice Research Team is dedicated to helping you sort through the myths and half-truths of the cosmetic industry. With their expert advice, you’ll have the facts you need to take the best possible care of your skin.

585630-IIS2 v1.0.0.431 10/4/2015 4:37:01 PM
Skip to Top of Page

Free Email Sign-Up

- (Less Frequent)
- Expert Advice Newsletter (twice a month)
- New reviews on Beautypedia (twice a month)

Birthday (Optional)



Paula’s Choice Auto Delivery is a quick and convenient way to make sure you never run out of your favorite products again. Just select the products you can’t live without, choose how often you’d like them delivered and we’ll take care of the rest.

20% Off One Item + Free Shipping on $50+