Lichen planus: A common skin disease with small itchy pink or purple spots on the arms or legs.

The lesions (abnormal areas) on the skin in lichen planus are typically polygonal, flat (hence, the term planus), and pruritic (itchy). Occurs characteristically on the wrists, shins, lower back and genitalia. Involvement of the scalp may lead to hair loss.

The cause(s) of lichen planus are unknown. However, it can be triggered by the use of certain drugs (thiazide diuretics, phenothiazines, antimalarials).

Treatment is with topical corticosteroids. In most cases, the disease spontaneously regresses 6 months to 2 years after onset.