Merck Manual

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Quick Facts



The Manual's Editorial Staff

Last full review/revision Aug 2019
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Topic Resources

What is psoriasis?

Psoriasis is a chronic (long-lasting) skin disease that causes raised, red patches on your skin. The raised patches may have silvery scales.

  • Psoriasis is common and can run in families

  • Light-skinned people get psoriasis more often than dark-skinned people

  • Psoriasis usually starts when you're between ages 16 and 22 years or between ages 57 and 60 years

  • The patches can be large or small, and they can appear anywhere on your body, especially your elbows, knees, and scalp

  • Psoriasis can’t be cured, but the patches can go away for long periods of time and then come back

  • Doctors treat psoriasis with phototherapy (shining a UV light on your skin to heal it) and medicine

What causes psoriasis?

Doctors aren’t sure exactly what causes psoriasis. It may be a problem with your immune system (the cells, tissues, and organs that protect your body from illness and infection). Psoriasis tends to run in families. That means if you have psoriasis some of your relatives may also have psoriasis.


Psoriasis tends to come and go, although some patches may never go away. Flare-ups are when it comes back or gets worse. Flare-ups can be triggered by:

  • Skin injuries

  • Sunburn

  • Infections, such as colds and strep throat

  • Winter weather

  • Drinking alcohol

  • High stress levels

  • Certain medicines

Flare-ups are more common for people who are overweight, smoke tobacco, or have HIV infection.

What are the symptoms of psoriasis?

  • One or more raised, red skin patches with silver scales on your scalp, elbows, knees, low back, or buttocks

  • Skin patches may also show up on your eyebrows, armpits, belly, around the anus, or between your buttocks

  • Sometimes, your fingernails are misshapen, thick, and pitted

The skin patches may:

  • Clear up after a few months

  • Stay the same

  • Grow bigger

  • Develop on other parts of your body

  • Go away and not come back for years

The patches may itch or hurt. You may be bothered by the way they look.

Some people with psoriasis have other symptoms, such as swollen, painful joints or pus-filled blisters.

How can doctors tell if I have psoriasis?

Doctors usually can tell you have psoriasis by the way your skin patches look. If doctors aren't sure, they'll take a sample of your skin to look at under a microscope (biopsy).

How do doctors treat psoriasis?

Doctors will give you one or more treatments:

  • Medicines to put on your skin

  • Phototherapy (UV light that is shined on your skin to heal it)

  • Medicine given as pills or a shot—some of these medicines have side effects so they're only used to treat severe psoriasis

NOTE: This is the Consumer Version. DOCTORS: Click here for the Professional Version
Click here for the Professional Version
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