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.
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. Its use in both rinse-off and leave-on cosmetics products is restricted to low amounts as a means to avoid sensitivity 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.
References for this information:
Anais Brasileiros de Dermatologia, 2014, issue 3, page 527
Cosmetics & Toiletries, 2014, issue 2, page 24