DefinitionLichen planus is a disease in which there is an itchy, swollen rash on the skin or in the mouth.Causes, incidence, and risk factorsThe exact cause of lichen planus is unknown. However, it is likely to be related to an allergic or immune reaction.Risks include:Exposure to medications, dyes, and other chemical substances (including gold, antibiotics, arsenic, iodides, chloroquine, quinacrine, quinide, phenothiazines, and diuretics)Disorders such as hepatitis CLichen planus generally affects middle-aged adults. It is less common in children.SymptomsMouth lesionsTender or painful (mild cases may have no discomfort)Located on the sides of the tongue or the inside of the cheekOccasionally located on the gumsPoorly defined area of blue-white spots or "pimples"Lines of lesions that form