Lichen planus is an uncommon disorder involving a recurrent, itchy, inflammatory rash or lesion on the skin or in the mouth. The exact cause is unknown, but the disorder is likely to be related to an allergic or immune reaction.

The disorder has been known to develop after exposure to potential allergens such as medications, dyes, and other chemical substances. Symptoms are increased with emotional stress, possibly because of changes in immune system during stress.

Lichen planus generally occurs at or after middle age. It is less common in children. The initial attack may last for weeks to months, resolve, then recur for years.

Lichen planus may be associated with several other disorders, most notably Hepatitis C.

Chemicals or medications associated with development of lichen planus include gold (used to treat rheumatoid arthritis), antibiotics, arsenic, iodides, chloroquine, quinacrine, quinidine, antimony, phenothiazines, diuretics such as chlorothiazide, and many others.