Methylchloroisothiazolinone deserves extra caution when you have seborrheic dermatitis. It is a preservative with a well-established contact-allergy concern, so a product containing it can add itching, redness or burning that feels much like a worsening flare. The ingredient list cannot tell you whether you are already sensitised.

This is not a Malassezia-feeding warning, and preserving a formula does not make Methylchloroisothiazolinone a seb derm treatment. Product format and exposure matter, but people with a known isothiazolinone allergy should follow their clinician's avoidance advice. Otherwise, stop a product that repeatedly makes affected skin itch, sting or redden.