Merck Manual

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Shinjita Das

, MD, Harvard Medical School

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Topic Resources

Psoriasis is a chronic, recurring disease that causes one or more raised, red patches that have silvery scales and a distinct border between the patch and normal skin.

  • A problem with the immune system may play a role, and some people are genetically predisposed to psoriasis.

  • Characteristic scales or red patches can appear anywhere on the body in large or small patches, particularly the elbows, knees, and scalp.

  • This disease is treated with a combination of drugs applied to the skin, exposure to ultraviolet light (phototherapy), and drugs taken by mouth or given by injection.

Psoriasis is common and affects about 1 to 5% of the population worldwide. Light-skinned people are at greater risk, whereas blacks are less likely to get the disease. Psoriasis begins most often in people aged 16 to 22 years and aged 57 to 60 years. However, people in all age groups and races are susceptible.

The patches of psoriasis occur because of an abnormally high rate of growth of skin cells. The reason for the rapid cell growth is unknown, but a problem with the immune system is thought to play a role. The disorder often runs in families, and certain genes are associated with psoriasis.


Plaque psoriasis, the most common type of psoriasis, usually starts as one or more small red, silvery, shiny patches (plaques) on the scalp, elbows, knees, back, or buttocks. The eyebrows, underarms, navel, the skin around the anus, and the cleft where the buttocks meet the lower back may also be affected. Many people with psoriasis may also have deformed, thickened, and pitted nails.

The first patches may clear up after a few months or remain, sometimes growing together to form larger patches. Some people never have more than one or two small patches, and others have patches covering large areas of the body. Thick patches or patches on the palms of the hands, soles of the feet, or skinfolds of the genitals are more likely to itch or hurt, but many times the person has no symptoms. Although the patches do not cause extreme physical discomfort, they are very obvious and often embarrassing to the person. The psychologic distress caused by psoriasis can be severe.

Psoriasis persists throughout life but may come and go. Symptoms of psoriasis are often diminished during the summer when the skin is exposed to bright sunlight. Some people may go for years between occurrences.

About 5 to 30% of people with psoriasis develop arthritis (psoriatic arthritis). Psoriatic arthritis causes joint pain and swelling.


Psoriasis may flare up for no apparent reason or as a result of a variety of circumstances. Flare-ups often result from conditions that irritate the skin, such as minor injuries and severe sunburn. Sometimes flare-ups occur after infections, such as colds and strep throat. Flare-ups are more common in the winter, after drinking alcohol, and after stressful situations. Many drugs, such as antimalarial drugs, lithium, angiotensin-converting enzyme (ACE) inhibitors, terbinafine, interferon-alpha, and beta-blockers, can also cause psoriasis to flare up. Flare-ups are also more common among people who are obese, infected with the human immunodeficiency virus (HIV), or smoke tobacco.

Uncommon types of psoriasis

Some uncommon types of psoriasis can have more serious effects.

Erythrodermic psoriasis causes all of the skin on the body to become red and scaly. This form of psoriasis is serious because, like a burn, it keeps the skin from serving as a protective barrier against injury and infection.

Pustular psoriasis is another uncommon form of psoriasis. In this form, large and small pus-filled blisters (pustules) are scattered widely on the body.

Palmoplantar psoriasis is a form of pustular psoriasis in which pustules occur primarily on the hands and feet. It is sometimes called palmoplantar psoriasis of the palms and soles.


  • A doctor's evaluation

  • Sometimes skin biopsy

Doctors base the diagnosis of psoriasis on how the scales and plaques look and where they appear on the body.

Sometimes, doctors take a sample of skin tissue and examine it under a microscope to rule out other disorders (such as skin cancer).


  • Topical drugs

  • Phototherapy

  • Immunosuppressants

  • Other drugs

Many drugs are available to treat psoriasis. Most often, a combination of drugs is used, depending on the severity and extent of the person's symptoms.

Topical drugs

Topical drugs (drugs applied to the skin) are used most commonly. Nearly everyone with psoriasis benefits from skin moisturizers (emollients).

Other topical agents include corticosteroids, often used together with calcipotriene (also called calcipotriol), which is a form of vitamin D, or coal tar.

Tacrolimus and pimecrolimus are used to treat psoriasis that appears on delicate skin (such as on the face or groin or in skinfolds). Tazarotene or anthralin may also be used.

Very thick patches can be thinned with ointments containing salicylic acid, which make the other drugs more effective.

Many of these drugs are irritating to the skin, and doctors must find which ones work best for each person.


Phototherapy (exposure to ultraviolet light) also can help clear up psoriasis for several months at a time (see Phototherapy: Using Ultraviolet Light to Treat Skin Disorders). Phototherapy is often used in combination with various topical drugs, particularly when large areas of skin are involved. Traditionally, treatment has been with phototherapy combined with the use of psoralens (drugs that make the skin more sensitive to the effects of ultraviolet light). This treatment is called PUVA (psoralen plus ultraviolet A).

Many doctors are now using narrowband ultraviolet B (NBUVB) treatments, which are as effective as PUVA. However, NBUVB treatments are done without psoralens and therefore do not have the same side effects, such as extreme sensitivity to sunlight.

Doctors can also treat specific patches of the skin directly by using a laser that focuses ultraviolet light (called excimer laser therapy).


Immunosuppressants are drugs that intentionally weaken (suppress) the immune system to keep it from making psoriasis worse. These drugs can be taken by mouth or given by injection. Immunosuppressants can reduce the body's ability to fight infections.

Cyclosporine can be used to treat severe psoriasis. This drug may cause high blood pressure and damage the kidneys.

Mycophenolate commonly causes gastrointestinal problems and bone marrow suppression (decreased production of red blood cells, white blood cells, and platelets). It may also increase the risk of lymphoma and other cancers.

Methotrexate decreases inflammation in the body and interferes with the growth and multiplication of skin cells. Doctors use methotrexate to treat people whose psoriasis is severe or does not respond to less harmful forms of therapy. Liver damage and impaired immunity are possible side effects.

Other drugs

Other drugs may be given to treat serious forms of psoriasis and psoriatic arthritis.

Acitretin is particularly effective in treating pustular psoriasis but often raises fat (lipid) levels in the blood and might cause problems with the liver and bones as well as reversible hair loss. This drug is given by mouth. It can cause severe birth defects and should not be taken by women who may become pregnant. Women should wait at least 2 years after their last dose of acitretin before attempting pregnancy.

Biologic agents are made from living organisms and inhibit certain chemicals involved in the immune system. They include etanercept, adalimumab, infliximab, ustekinumab, secukinumab, brodalumab, ixekizumab, tildrakizumab, risankizumab, guselkumab, and tofacitinib. All of these drugs are given by injection except tofacitinib, which is taken by mouth. Apremilast is another option and it is taken by mouth. They tend to be the most effective drugs for severe psoriasis, but long-term safety is not clear.


Phototherapy: Using Ultraviolet Light to Treat Skin Disorders

Exposure to sunlight is helpful for certain skin disorders. One component of sunlight―ultraviolet (UV) light―is responsible for this effect. UV light has many different effects on skin cells, including altering the amounts and kinds of chemicals they make and causing the death of certain cells that can be involved in skin diseases.

The use of UV light to treat disease is called phototherapy. Psoriasis and atopic dermatitis are the disorders most commonly treated with phototherapy.

Because exposure to natural sunlight varies in intensity and is not practical for a large part of the year in certain climates, phototherapy is nearly always done with artificial UV light. Treatments are given in a doctor's office or in a specialized treatment center.

UV light, which is invisible to the human eye, is classified as A, B, or C, depending on its wavelength. Ultraviolet A (UVA) penetrates deeper into the skin than ultraviolet B (UVB). UVA or UVB is chosen based on the type and severity of the person's disorder. Ultraviolet C is not used in phototherapy. Some lights produce only certain specific wavelengths of UVA or UVB (narrowband therapy), which are used to treat specific disorders. Narrowband therapy helps limit the sunburn–like effect caused by phototherapy.

Phototherapy is sometimes combined with the use of a psoralen. The combination of a psoralen plus UVA is known as PUVA therapy. Psoralens are drugs that may be taken by mouth before treatment with UV light. Psoralens sensitize the skin to the effects of UV light, allowing shorter, less intense exposure.

Side effects of phototherapy include pain and reddening similar to sunburn with prolonged exposure to UV light. UV light exposure also increases the long-term risk of skin cancer, but the risk is small for brief courses of treatment. Psoralens often cause nausea and extreme sensitivity to sunlight. In addition, because psoralens enter the lens of the eye, UV-resistant sunglasses must be worn for at least 12 hours after undergoing PUVA therapy.

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