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. 
- 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.