Lichen planus is caused by an immune system attack on the skin or mucous membranes, but the exact trigger that starts this process remains unknown in most cases. What researchers do understand is the mechanism: certain white blood cells mistakenly target and destroy cells in the outer layer of skin, producing the itchy, purplish, flat-topped bumps characteristic of the condition. The condition affects roughly 1% of the general population and is most common in adults over 40.
How the Immune System Drives the Disease
Lichen planus is a T-cell mediated disease, meaning a specific branch of the immune system is responsible. Two types of immune cells play central roles. Helper T cells (Th1 and Th17 subtypes) set the stage by releasing inflammatory signaling molecules, particularly interferon-gamma and interleukin-17. These signals recruit and activate a second group, cytotoxic T cells, which do the actual damage.
Those cytotoxic cells destroy skin cells through several pathways. They release molecules like perforin and granzyme B that punch holes in skin cells and trigger their programmed death. They also use surface-level signaling (through what’s called the Fas-FasL pathway and TNF-alpha receptors) to force skin cells to self-destruct. The result is the inflammation, color changes, and raised lesions that define lichen planus.
The process is self-reinforcing. Damaged skin cells release proteins that attract more immune cells, which cause more damage. This is why lichen planus tends to persist and flare rather than resolve quickly on its own.
Genetic Predisposition
Some people appear genetically more susceptible to lichen planus than others. Research has linked the condition to several variations in HLA genes, which help the immune system distinguish the body’s own cells from foreign invaders. Specific variants (HLA-DR1, DR2, DR3, DR9, and DR10) have been associated with increased risk across different ethnic populations. In one study of Mexican Mestizo patients, carrying the HLA-DRB1*0101 variant made a person more than five times as likely to develop lichen planus compared to people without it.
Having a genetic predisposition doesn’t guarantee you’ll develop the condition. It likely means your immune system is more prone to the misfiring that triggers it, especially when combined with an environmental trigger.
Hepatitis C and Viral Connections
The strongest known disease association with lichen planus is hepatitis C. A large meta-analysis across 143 studies found that about 9.4% of lichen planus patients tested positive for hepatitis C, a rate significantly higher than in the general population. The relationship appears to go both ways: people with hepatitis C are more likely to develop lichen planus, and people with lichen planus are more likely to have undiagnosed hepatitis C.
The connection likely involves the virus altering how the immune system behaves, pushing it toward the kind of overactive T-cell response that attacks skin cells. In areas where hepatitis C is more common, doctors often screen lichen planus patients for the virus.
Medications That Can Trigger It
Dozens of medications can cause a reaction that looks identical to lichen planus, called a lichenoid drug eruption. The rash typically appears weeks to months after starting the medication and often resolves after stopping it. The most commonly implicated drug categories include:
- Blood pressure medications: calcium channel blockers (like amlodipine), ACE inhibitors (like enalapril), angiotensin receptor blockers (like losartan), and thiazide diuretics
- NSAIDs: including naproxen and other common anti-inflammatory painkillers
- Diabetes medications: including glimepiride, metformin, and chlorpropamide
- Antimalarials and gold-based treatments
- Tuberculosis drugs: pyrazinamide, rifampicin, isoniazid, and ethambutol
- Proton pump inhibitors used for acid reflux
- Newer biologic and immunotherapy drugs: including checkpoint inhibitors and some biologics used for eczema
Lichenoid drug eruptions have also been documented following COVID-19 vaccination, particularly with mRNA vaccines like the Pfizer/BioNTech vaccine, though these cases are uncommon.
Dental Materials and Oral Lichen Planus
For lichen planus that appears inside the mouth, contact allergy to dental materials is a recognized trigger. Mercury in amalgam (silver) fillings is the most studied culprit. Several studies have reported that oral lesions improved or fully resolved after amalgam fillings were replaced, particularly in patients who tested positive for mercury allergy on a patch test.
Other dental materials that have been investigated as potential triggers include gold alloys, epoxy resins, and composite filling materials. The relationship is somewhat controversial because not every patient with oral lichen planus near a filling has a true allergy, and removal doesn’t always help. Patch testing can identify which patients are most likely to benefit from having fillings replaced with alternative materials like ceramics, glass ionomers, or composite resins.
Stress and Cortisol
Psychological stress is consistently linked to lichen planus flares, and the connection appears to be more than anecdotal. A systematic review with meta-analysis found that every study examined reported significantly higher cortisol levels (the body’s primary stress hormone) in oral lichen planus patients compared to healthy controls. The more severe the lesions, the higher the cortisol tended to be, with the erosive (ulcerative) form showing the strongest association with emotional stress.
The biological pathway makes sense. Chronic stress disrupts the body’s hormonal balance through the hypothalamic-pituitary-adrenal axis, the system that regulates cortisol. This disruption alters immune function, specifically the production of inflammatory signaling molecules that drive the T-cell attack on skin cells. Stress doesn’t cause lichen planus on its own, but in someone who is already predisposed, it can initiate or worsen flares.
Thyroid Disease and Other Conditions
People with lichen planus are more likely to have certain other health conditions, particularly thyroid disease. In a study of 334 patients with a scalp-specific form of lichen planus, nearly a quarter had hypothyroidism, compared to about 13% of controls. Hashimoto’s thyroiditis, an autoimmune thyroid condition, appeared in 6.3% of patients and in none of the controls.
Interestingly, the same study did not find statistically significant links with most other autoimmune diseases like lupus, rheumatoid arthritis, or celiac disease. The thyroid connection stands out, possibly because thyroid autoimmunity and lichen planus share similar immune pathways. Other conditions that appeared more frequently in lichen planus patients included vitamin D deficiency, depression, type 2 diabetes, and high cholesterol.
Cancer Risk With Oral Lichen Planus
One concern many people have after being diagnosed with oral lichen planus is whether it can become cancerous. A long-term Italian study followed over 3,100 patients with confirmed oral lichen planus for up to 33 years and found that 2.58% developed oral squamous cell carcinoma. The average time between the initial lichen planus diagnosis and cancer development was about eight and a half years. This risk is low in absolute terms but high enough that regular monitoring of oral lesions is standard practice, particularly for the erosive type that causes open sores or ulcers in the mouth.