The cause of lichen planus has not yet been determined. However, the condition is generally considered to be an autoimmune disease, in which the antibodies of the immune system attack the body’s own tissues. Some physicians believe that several types of chemicals can trigger an immune system response in some people, causing lichen planus. Exposure to the following chemicals may be associated with lichen planus:

• Gold (used to treat rheumatoid arthritis)
  
  
• Bismuth
  
  
• Arsenic
  
  
• Iodides
  
  
• Chloroquine and quinine (antimalarial drugs)
  
  
• Antimony
  
  
• Phenothiazines (antipsychotic medications)
  
  
• Antibiotics
  
  
• Diuretics (drugs that promote the excretion of urine)
  
  
• Amalgam (a mixture of equal parts liquid mercury and alloy powder containing silver, tin, and copper used in dental fillings)

Some patients have a family medical history of lichen planus, although it does not seem to be inherited. Psychological factors (e.g., fatigue, stress) do not cause the disease, but they may make it worse.

Lichen planus is also associated with certain liver disorders, particularly hepatitis C. The association is not well understood, but individuals with these conditions are more likely to have lichen planus.

In addition, patients with mouth sores from lichen planus have a slightly increased risk of developing oral cancer. Skin involvement, however, does not increase the patient’s risk of cancer.