Toluene-2,5-Diamine Sulfate: Allergen of the Year 2025.

Toluene-2,5-diamine sulfate (PTDS), also known as toluene diamine sulfate or 2,5-diaminotoluene sulfate, is an aromatic amine that can be used as a paraphenylenediamine (PPD) hair dye alternative. Patients may develop contact allergy to PTDS, and it is often omitted from standard patch test screening series, possibly leading to underdiagnosis. The American Contact Dermatitis Society named PTDS the Allergen of the Year for 2025, aiming to raise awareness of its dual role as both an allergen and an alternative for some PPD-allergic individuals.