What causes Lichen planus?

The cause of LP is unknown. Etiologic hypotheses include hypersensitivity reactions, viral infection (particularly hepatitis C), or autoimmune mechanisms. Case reports have linked LP to viral hepatitis, neurologic disease, and severe psychic trauma. Other autoimmune diseases such as vitiligo, inflammatory bowel disease, and autoimmune thyroiditis are seen with increased frequency in those with LP. Lichenoid eruptions  (otherwise identical to idiopathic LP) may be seen as a reaction to drugs or as a manifestation of graft-versus-host disease following allogeneic bone marrow transplantation. Oral disease and, to a lesser degree, cutaneous LP have been associated with hypersensitivity to dental amalgams. In actuality, most LP occurs in otherwise healthy people without an identifiable cause.