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 may solely involve the penis but can also affect the skin more broadly, and can also affect hair follicles, the nail unit, and mucous membranes. See oral lichen planus for oral mucosal presentation and lichen planopilaris for scalp presentation.

Clinically, LP presents with flat-topped, reddish to purple papules. Penile LP most commonly presents on the glans and is often annular in configuration. Papules and plaques can erode, resulting in significant pain, scarring, and impairment of function.

Drugs causing LP-like eruptions (lichenoid drug reactions) include antihypertensives (angiotensin-converting enzyme [ACE] inhibitors: captopril and enalapril; beta blockers: propranolol and labetalol), thiazide diuretics, antimalarials (quinidine and hydroxychloroquine), penicillamine, NSAIDs, griseofulvin, tetracycline, antiepileptics, and many other drugs.

LP can spontaneously resolve, usually after a year, or follow a remitting or chronic course. LP of mucosal sites tends to be more chronic and treatment resistant than cutaneous LP.