Lichen planus (LP) is a condition in which autoreactive T lymphocytes attack basal keratinocytes in the skin, mucous membranes, hair follicles, and/or nail units. The etiology is unclear, but viruses, medications, and contact allergens have all been implicated. LP is most common in adults in the fourth to sixth decades of life, but it may occur at any age. There is no known predilection for either sex or ethnicity. LP is estimated to occur in less than 1% of the population.

Clinically, patients present with pruritic, flat-topped, pink-to-purple papules that are localized most commonly along the volar wrists, shins, presacral area, and hands but may be widespread. Oral LP and/or LP involving the genitalia can occur in isolation or in patients with more widespread cutaneous disease. Mucosal disease may more rarely affect the conjunctivae, anus, tonsils, larynx, and the mucosal surfaces throughout the gastrointestinal tract.

Lichen planopilaris, a variant of LP affecting the follicular unit, presents with perifollicular erythema and scaling and leads to scarring alopecia. (Frontal fibrosing alopecia is a variant seen in older women. Another rare variant is Graham-Little-Piccardi-Lassueur syndrome.) LP can also affect the nail matrix, resulting in fissuring, longitudinal ridging, and lateral thinning of the nails. Otic LP (involving the external auditory meatus and tympanic membrane) is a rare variant.

Certain medications cause an LP-like eruption. Culprits include captopril, enalapril, labetalol, propranolol, methyldopa, calcium channel blockers, NSAIDs, chloroquine, hydroxychloroquine, quinacrine, thiazide diuretics, etanercept, infliximab, penicillamine, quinidine, and gold salts.

LP has been described in association with hepatitis C, predominantly in certain geographical areas (Japan and the Mediterranean region). Hepatitis B vaccination, as well as exposure to other bacteria and viruses, has also been associated with LP in the literature. Oral LP may occur on mucosal surfaces apposed to amalgams and other dental restorative materials.