In most cases, the cause of lichen planus is not known. It is often though to be the result of an autoimmune process where the body’s immune system attacks its own tissues. When lichen planus is caused by a triggering agent such as hepatitis B or C, it is called a lichenoid reaction.
Medications to treat high blood pressure, heart disease, and arthritis can cause lichen planus-type rashes that are allergic reactions to those drugs. Lichen planus is not contagious.