What exactly causes Lichen planus has not been clearly understood till date but various theories have existed as to what can trigger this condition in some people and not in others. Some of the commonly cited reasons include:

An allergic or immune reaction to a medication or other substance has been strongly advocated as the cause for this condition. It's considered to be an auto-immune disease in which the body's protective cells attack the body's own tissues mistaking them to be foreign in nature.

Certain cases have been linked to infections such as hepatitis C

Quite a few drugs such as those containing arsenic, bismuth, gold can cause a reaction like the rash of lichen planus

Chemicals such as those used in the development of color photographs can also produce a similar rash

Long-term use of drugs like quinacrine or quinidine (anti-malarial drug) may produce hypertrophic lichen planus. Some other drugs that can trigger these kind of lesions include NSAIDs (Non-steroidal anti-inflammatory drugs), certain antibiotics (Dapsone, Penicillamine, Tetracycline, Sulfonamide), medicines to treat high blood pressure (Captopril, Propranolol, Methyldopa, Enalapril), Furosemide, Allopurinol, etc.

Stress - Intense and prolonged emotional stress such as sadness, anxiety, etc. can trigger an abnormal functioning of the immune system that can lead to Lichen planus.

Genetic cause

Amalgam dental fillings may trigger allergic reactions giving rise to oral lesions of lichen planus