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 < 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 cutaneous disease. 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 that is seen in older women. Another rare variant is the Graham-Little-Piccardi-Lassueur syndrome.) LP can also affect the nail matrix, resulting in fissuring, longitudinal ridging, and lateral thinning of the nails.

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 Mediterranean regions). 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.