There are two clear active ingredients that are cleared by the FDA for the treatment of psoriasis on your scalp: salicylic acid and cold tar. Nobody these days uses cold tar shampoos because of the smell, but salicylic acid shampoos are quite nice. The way salicylic acid works is that it’s an acid that breaks the bonds between the individual skin cells [and turns them into] flakes. So the flakes become smaller, and they come off in smaller parts.