1 corticosteroids: These help to reduce the inflammation of a lichen planus rash, and can be topical, oral, or given as an injection. 2  antihistamines: These also reduce inflammation and may be particularly helpful if your rash is triggered by an allergen.he most common type of lichen planus affects the skin. The lesions appear and spread over the course of several weeks and usually clear up within six months to a year and a half. Less commonly the lesions can occur on mucous membranes, genitalia, nails, and the scalp.