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 of the male genitalia typically occurs in uncircumcised adult males, with a reported median age of onset at 51 years. In the vast majority of reported cases, only the genitalia are involved.
However, sometimes the skin is affected more broadly, and LP can also affect hair follicles, nail units, and mucous membranes. See oral LP for oral mucosal presentation, and see lichen planopilaris for scalp presentation.

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

Drugs causing LP-like eruptions (lichenoid drug reactions) include antihypertensives (angiotensin-converting enzyme [ACE] inhibitors such as captopril and enalapril; beta blockers such as 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.