Methylisothiazolinone, or MI, deserves extra caution for skin affected by seborrheic dermatitis because it has a well-documented contact-sensitisation concern. For a leave-on product, or if you have known isothiazolinone allergy, avoidance is the clearer choice.

MI preserves the product in its container; that does not mean it safely treats Malassezia on skin. Contact allergy can look like worsening dermatitis, so repeated reactions need product-specific assessment rather than a formula score. Stop use and seek allergy advice if a reaction persists.