The cause of lichen planus is not known, as of 2015, states Mayo Clinic. It is believed to be an auto-immune disease, which means that the immune system of a patient attacks parts of the body as if they are foreign invaders.
Lichen planus usually manifests as purplish, sometimes itchy, bumps on the skin, explains Mayo Clinic. Lichen planus can appear in the mouth or vagina as white patches with sores. It is most frequently found in middle-aged adults, according to the American Academy of Dermatology.
Some things are known to contribute to lichen planus, states the AAD. Some medications, such as diuretics and anti-malarial drugs, cause a rash similar to lichen planus. Metal fillings in the teeth can cause lichen planus of the mouth. One form, known as familial bullous lichen planus, may run in families. People with hepatitis C seem to acquire lichen planus with a greater frequency than people without the illness.
Other possible causes and triggers of lichen planus include certain vaccines and drugs, reports Mayo Clinic. The hepatitis B vaccine and the flu vaccine may bring on the condition. Some nonsteroidal drugs used to reduce inflammation, such as ibuprofen and naproxen, may also trigger lichen planus. The condition can also be triggered by specific pigments, chemicals and metals.