Preservative that’s generally recommended for use only in rinse-off products such as cleansers or shampoos. Methylisothiazolinone is known to be sensitizing when used in leave-on products.
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:
JAMA Dermatology, January 2016, pages 67-72
Anais Brasileiros de Dermatologia, 2014, issue 3, page 527
Actas-dermo-sifiliograficas, November 2014, pages 854-859
Cosmetics & Toiletries, 2014, issue 2, page 24
CMAJ, October 2013, page E71