Lichen planus is a rash that typically occurs on the lower legs, ankles, forearms, and wrists. The spots that make up the rash of lichen planus are purple, slightly scaly bumps. It is often itchy. The rash can come and go, but is usually chronic.
When lichen planus occurs in the mouth, it is called oral lichen planus. It appears most commonly on the insides of the cheeks, gums, and tongue. In the mouth, lichen planus can look like white lacy streaks, fluid-filled blisters, or sores or ulcers that are particularly painful.
Sometimes the cause of lichen planus is unknown. However, lichen planus-type rashes can occur as an allergic reaction to medications for high blood pressure, heart disease and arthritis. Lichen planus, on rare occasion, is associated with hepatitis C virus infection. Reactions to dental amalgam fillings may contribute to the oral lesions of lichen planus, and studies have shown that the lesions resolve after the fillings are replaced with another material.