Lichen Planus: What Causes It, 6 Signs a Dermatologist Looks For, How It Differs from Psoriasis and Drug Rash, and Treatment

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Lichen planus is a chronic inflammatory skin condition caused by the immune system, in which white blood cells attack the lower layer of your own skin cells, producing very itchy, dark purple raised bumps. It can appear not only on the skin but also in the mouth, nails, scalp and genitals, and each site has different consequences, especially the scalp, where it can cause permanent scarring hair loss, and the mouth, where there is a small risk of cancer.

What often confuses patients is that lichen planus can look like psoriasis, a drug rash or a fungal rash, and in some cases it is triggered by a medicine you are taking without you knowing. In this article, a dermatologist explains what is characteristic of a lichen planus rash, how to tell it apart from other conditions, and how it is treated.

What is lichen planus?

Lichen planus is not contagious and is not caused by poor hygiene or infection. It happens when the immune system works abnormally and triggers inflammation at the junction between the upper and lower layers of the skin, damaging skin cells and causing a long-lasting rash.

It is seen more often in middle-aged adults than in children, and it runs a chronic course. Skin rashes often improve on their own over several months to 1–2 years, but the types affecting the mouth, nails or scalp tend to be long-lasting and harder to treat.

6 features of a lichen planus rash

The skin rash of lichen planus has features that doctors remember as "purple, flat, polygonal, itchy":

Very itchy (Pruritic) — the main symptom, which often affects quality of life
Purple (Purple/violaceous) — different from an ordinary red rash
Angular borders (Polygonal) — not smoothly round
Flat surface (Planar/flat-topped) — raised bumps with flat tops
Papules or plaques (Papules/Plaques) — often on the inner wrists, arms, ankles and shins
Fine white lines on the rash surface (Wickham's striae) — a small white lacy network on top of the bumps, an important clue for diagnosis

The rash also tends to appear along scratches or scrapes in a line (Koebner phenomenon), so scratching makes the rash spread further.

Lichen planus can appear in many places, and each place is different

What makes lichen planus more important than people think is that it is not limited to the skin:

In the mouth (Oral lichen planus) — white patches or lacy white lines on the inner cheeks, tongue or gums. Some people have red, painful sores. This type is long-lasting and carries a small risk of becoming oral cancer, so it needs follow-up.
On the nails — thin nails, lengthwise ridges, nail wasting, or scar tissue pulling at the nail (pterygium) that can damage the nail permanently (it is hard to tell apart from nail fungus — see our article on "nail fungus" for more).
On the scalp (Lichen planopilaris) — causes patchy hair loss with a scarred scalp, and hair lost to scarring will not grow back, so treatment should start early to protect the remaining hair.
On the genitals — a long-lasting rash or sores that can be painful and affect daily life.

Because some of these sites can suffer permanent damage, early diagnosis and treatment are important.

What causes lichen planus, and who is at risk?

The exact cause is not clear, but it is a reaction of the T-cell immune system attacking the body's own skin cells. Associated factors worth knowing about include:

Certain medicines (lichenoid drug eruption) — some blood pressure medicines, diuretics, antimalarials, anti-inflammatory drugs, NSAIDs and others can trigger a rash that looks like lichen planus. In this case it usually improves once the causative drug is stopped.
Hepatitis C virus — an association has been seen in some populations, and a doctor may consider testing in suitable cases.
Dental filling materials or contact allergy — linked to oral lichen planus in some people.
Stress and friction or injury to the skin — may act as triggers.

Lichen planus is not contagious and is not directly inherited.

Lichen planus vs psoriasis vs drug rash — how to tell them apart

This is the part where diagnosis is often missed, because many chronic rashes look alike. A dermatologist tells them apart as follows:

Lichen planus — purple, flat, angular, very itchy bumps with white lines (Wickham), usually on the inner wrists or ankles, and there may be lesions in the mouth or nails as well.
Psoriasis — thick red plaques with silvery scale, usually on the elbows, knees and scalp, with pitted nails, and white lines like Wickham's are uncommon.
Lichenoid drug eruption — looks very much like lichen planus, but there is a history of starting a new medicine before the rash appeared, and it usually improves when the drug is stopped.
Fungal rash / tinea versicolor — ring-shaped with a clear edge and scaling, and fungus can be found on testing, unlike lichen planus, which has no infection.
Eczema (atopic dermatitis) — also itchy, but the skin is usually dry and red in uneven patches, without purple angular bumps showing Wickham's lines.

The key point is that a chronic, itchy, purple rash with white lines, especially with mouth or nail involvement, should make you think of lichen planus, and a biopsy can confirm it clearly.

Dark marks after the rash in Thai skin

A common concern for patients with skin tones like Thai skin is that even after the lichen planus rash has settled, it often leaves dark marks (post-inflammatory hyperpigmentation) for a long time, especially after rashes that lasted a long time or were scratched. These marks fade gradually on their own but take several months to a year or more. Controlling inflammation early, not scratching and protecting from the sun can reduce dark marks. Any treatment for remaining marks should be done under a doctor's guidance.

How is lichen planus diagnosed?

Diagnosis relies on assessment by a dermatologist together with additional tests:

Examining the rash and dermoscopy — seeing Wickham's white lines helps confirm it
Skin biopsy — shows immune cells lined up along the junction of the skin, which is characteristic
Reviewing current medicines — to rule out a lichenoid drug rash
Further tests as appropriate — for example, screening for hepatitis in some people, and checking the mouth, nails and scalp as well

Treatment approaches for lichen planus

Treatment is individualized depending on the location and severity, and is assessed and prescribed by a doctor:

Topical medicines — medium to high potency corticosteroids are the main treatment for skin rashes, and calcineurin inhibitors (such as tacrolimus) are often used in the mouth and genital area
Injections into the lesion — steroid injections for thick lesions or on the scalp
Light treatment (phototherapy) — for rashes spread over the body
Oral medicines — short-term steroids, acitretin, methotrexate or cyclosporine for severe or resistant cases
Care for special sites — in the mouth, the focus is on controlling inflammation and monitoring for cancer; on the scalp, treatment should start early to protect hair; and if it is a drug rash, the causative drug must be stopped or changed together with the doctor who prescribed it

The goal is to calm the inflammation, reduce itching and prevent permanent damage to the nails and scalp. Continuing treatment and following up with your doctor are important.

When to see a doctor

See a dermatologist for a clear diagnosis if you have any of the following:

A chronic itchy purple rash that has lasted a long time or is spreading, and has not improved with self-treatment
Chronic sores or white patches in the mouth, especially sores that do not heal, are painful or have a lump — see a doctor to watch for oral cancer
Patchy hair loss with a red or scarred scalp — start treatment early before the hair loss becomes permanent
Nails that are misshapen, ridged or have scar tissue along with a rash on the body
A rash that appears after starting a new medicine, which may be a drug rash

Frequently asked questions about lichen planus

Is lichen planus contagious?

No. It is caused by your own immune system, not by germs, so it does not spread to other people and has nothing to do with cleanliness.

Can lichen planus be cured?

Skin rashes often improve on their own over several months to 1–2 years, but the types affecting the mouth, nails or scalp tend to be long-lasting and can come back. Treatment helps control symptoms and prevent permanent damage.

How is it different from psoriasis?

Lichen planus appears as purple, flat, angular, very itchy bumps with white lines (Wickham) and often has lesions in the mouth or nails, while psoriasis shows thick red plaques with silvery scale on the elbows, knees and scalp. A biopsy can tell them apart clearly when you are unsure.

Can lichen planus be caused by medicines?

Yes. Some medicines trigger a rash that looks like lichen planus (lichenoid drug eruption), which usually improves once the causative drug is stopped. Tell your doctor about every medicine you use, and do not stop any medicine on your own without consulting the doctor who prescribed it.

Does oral lichen planus carry a cancer risk?

The oral type carries a small risk of becoming oral cancer over the long term, especially the type with chronic sores, so you should follow up with a doctor regularly and see one promptly if you have a sore that does not heal, is painful or has a lump.

Do the dark marks after lichen planus go away?

Dark marks after the rash (PIH) in Thai skin usually fade on their own but take several months to a year or more. Controlling inflammation early, not scratching and protecting from the sun can reduce them. Any attempt to speed up fading should be done under a doctor's guidance.

Written by Dr. Patara Voravutinon, Dermatologist · Last reviewed August 2, 2026

The information in this article is for basic understanding only and does not replace diagnosis and treatment by a doctor. If your symptoms do not improve or you notice warning signs, consult a dermatologist for an examination and a treatment plan suited to you.