Several board-certified dermatologists told Allure that high-concentration skin-care acids, abrasive scrubs and harsh cleansers can trigger psoriasis flare-ups through a reaction called the Koebner phenomenon, in which trauma to the skin prompts the immune system to form new plaques in previously clear areas.
Omar Noor, a dermatologist based in New York City, said he generally advises psoriasis patients to limit leave-on products to 5% or less glycolic acid, about 8% or less lactic acid, or up to 10% mandelic acid; California-based dermatologist Divya Shokeen said L-ascorbic acid, the purest form of vitamin C, becomes "very irritating" at concentrations of 15% or higher. Physical exfoliants such as walnut-shell and apricot scrubs can cause small cuts that trigger new patches, Noor said, and he cautioned that microneedling and abrasive laser treatments carry similar risk, though he noted excimer laser treatments have been shown to help some patients with redness.