Preservative that is generally recommended only for use in rinse-off products such as cleansers or shampoos. Among the preservatives used in leave-on products, methylisothiazolinone is known to be sensitizing. This depends on the amount used, but as a general rule, if you see this ingredient in the middle of an ingredient list for a leave-on product it should be avoided, particularly if you have sensitive skin. Methylisothiazolinone is active against bacteria but has weak anti-fungal properties. Its use in rinse-off and leave-on cosmetic products is restricted to low amounts as a means of avoiding irritation while maintaining preservative efficacy. Along with methylchloroisothiazolinone, this preservative is a frequent allergen in leave-on products, particularly hair care and feminine hygiene (Sources: Contact Dermatitis, December 2012, pages 334-341 and November 2011, pages 276-285).