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! 
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. 
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.
- Scherrer M, Rocha V. Increasing trend of sensitization to Methylchloroisothiazolinone/methylisothiazolinone (MCI/MI). An Bras Dermatol. 2014;89(3):527.
- Flower C, Meredith E. Preserving with Methylisothiazolinone. Cosm & Toil. 2014;129(2):24.
- 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.
- Travassos A, Claes L, Boey L, Drieghe J, Goossens A. Non-fragrance allergens in specific cosmetic products. Contact Dermatitis. 2011;65(5):276-285.