Lichen planus is an inflammatory disease that usually affects the skin, the mouth, or both. It may affect the genital skin as well. The cause of lichen planus is not known. There are cases of lichen planus-type rashes occurring as allergic reactions to medications for high blood pressure, heart disease and arthritis. In those cases, identifying and stopping the use of the drug helps clear up the condition within a few weeks. Some people with lichen planus can also have hepatitis C and your dermatologist may want to check you for this. Lichen planus affects men and women equally, and occurs most often in middle-aged adults.