Lichen planus is a condition that forms an itchy rash on the skin or in the mouth. The exact cause of lichen planus is unknown. It may be related to an allergic or immune reaction. Risks for the condition include: exposure to medicines, dyes, and other chemicals (including gold, antibiotics, arsenic, iodides, chloroquine, quinacrine, quinide, phenothiazines, and diuretics), and diseases such as hepatitis C. Lichen planus mostly affects middle-aged adults. It is less common in children. Symptoms you may see include mouth sores that will sometimes form painful ulcers, skin sores that are itchy and have even sides (symmetrical) and sharp borders, dry mouth, hair loss, metallic taste in the mouth, and ridges in the nails (nail abnormalities). A skin lesion biopsy or biopsy of a mouth lesion can confirm the diagnosis. Blood tests may be done to rule out hepatitis. Lichen planus is usually not harmful. It usually gets better with treatment. The condition often clears up within 18 months but may come and go for years. If lichen planus is caused by a medicine you are taking, the rash should go away once you stop the medicine.