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What sulphur actually is.
Sulphur in dermatology is just the element — usually as precipitated or colloidal sulphur — applied topically. It has been in use for skin disease since antiquity, predating modern dermatology by centuries. The mechanism is a mix of mild keratolysis and direct antimicrobial activity, particularly against the demodex mite and some bacterial overgrowth patterns associated with inflammatory acne and rosacea-overlap conditions.
It is not a fashionable ingredient. It is also one of the few actives in this library that is reliably pregnancy-safe, gentler than BPO, and cheap enough to be a credible spot treatment. The trade-off is olfactory.