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What causes psoriasis?

What causes psoriasis?

Psoriasis is a common skin condition caused by skin cells growing and dividing too rapidly. It's not exactly known why this happens, but it's thought that the immune system is involved. The immune system seems to attack healthy skin cells by mistake, causing a series of events that leads to excess skin cell growth. Psoriasis is not contagious. It cannot be passed from person to person.


Psoriasis is caused by skin cells growing and dividing too rapidly. Skin cells normally live for around 30 days, from the time they start growing in the deeper layer of the skin until they die and flake off on the surface of the skin. In people with psoriasis, this process takes around three to five days. The excess skin cells lead to psoriatic plaques on the skin, which also tend to be dry and flaky. It's not fully known why this happens in the first place, but the immune system is thought to be important, alongside genetic factors.

People who have psoriasis often experience flare-ups, where their psoriasis gets worse for a few weeks or months. There are lots of possible triggers for flare-ups, including stress, throat infections, certain medications, and injuries to the skin.

In this series of articles centred around psoriasis, you can read about psoriasis treatment, psoriasis symptoms, and the causes of psoriasis - all written by one of our expert GPs.

The rest of this feature will take an in-depth look at the causes of psoriasis, as, at Patient, we know our readers sometimes want to have a deep dive into certain topics.

What causes psoriasis?

We don't fully understand why psoriasis affects some people, and not others. It's likely to be very complicated, and the exact reasons for someone to develop psoriasis probably differ from person to person. Theories on what causes psoriasis include:

Abnormal skin cell growth

This is ultimately what leads to areas of psoriasis on the skin. In psoriasis, cells in the outermost layer of the skin (the epidermis) grow much quicker than they normally do. This leads to immature, non-functioning cells on the surface of the skin, producing scaly, raised plaques of psoriasis.

Many of the symptoms of psoriasis can be directly linked to problems with the way these skin cells work. For example, they don't produce the normal moisturising oil that skin cells usually do, meaning that areas of psoriasis tend to be very dry.

Other things explain why abnormal skin cell growth happens in the first place.

Immune system problems

It's thought that immune system problems are a very important cause of psoriasis. The immune system can attack healthy skin cells by mistake. Immune cells (T-cells) seem to be really important. Activated T-cells live in the skin at areas of psoriasis, producing inflammation and releasing chemicals that cause skin cells to divide and grow rapidly.

Immune system problems also explain why some of the complications of psoriasis develop. For example, psoriatic arthritis is caused by the immune system mistakenly attacking the inside of the joints.

Many treatments for psoriasis work by reducing the immune system's activity.


Genetics seem to be important in psoriasis. Psoriasis runs in families - people with a family history of psoriasis are more likely to develop it themselves, although it's also possible to get psoriasis without any other family members being affected.

Various different genes have been linked with psoriasis. Most of these genes relate to how the immune system works. It's likely that some people inherit genes that make their immune system more likely to react in a certain way that leads to psoriasis.

Environmental triggers

Something else needs to happen for the immune system to start attacking healthy skin cells. It's thought that things in the environment can trigger this process, such as injuries to the skin.

What triggers psoriasis?

Psoriasis probably starts as a faulty immune response to something in the environment. People with psoriasis also find that certain things can trigger a psoriasis flare. Common psoriasis triggers include:

  • Injury to the skin - such as cuts, scrapes, and even tattoos, can trigger an immune response, and, in people prone to psoriasis, this immune response can go wrong. It's long been known that injury to normal areas of skin can cause psoriasis to appear there. This is called the Koebner phenomenon, first described by Heinrich Koebner in 1876 - and it can happen in other skin diseases too, like lichen planus.
  • Stress - can trigger a psoriasis flare, but psoriasis can also lead to stress, which can sometimes be a vicious circle.
  • Smoking - smokers are more likely to get psoriasis. Smoking makes psoriasis worse, and it also worsens many other things that go along with psoriasis, such as high blood pressure and heart problems. Stopping, or cutting down, smoking is helpful for psoriasis.
  • Alcohol - drinking excessive alcohol can make psoriasis worse.
  • Medications - certain medications can trigger psoriasis, such as:
  • Weather - dry and cold weather can trigger psoriasis for some people. It can cause skin to dry out, making it more vulnerable to injury.
  • Infection:
    • Infection with Streptococcus (for example Strep throat) can trigger psoriasis. Streptococcus infection is closely linked with guttate psoriasis - see psoriasis symptoms for more detail.
    • Psoriasis in people with HIV can be more severe and difficult to treat.
  • Pregnancy - some women find that psoriasis improves during pregnancy, but gets worse after giving birth.
  • Obesity and overweight - seem to be linked with a higher risk of psoriasis and tend to have more severe symptoms of psoriasis.
  • Intense sunlight can be a trigger for some people - though some people find the sun can help their symptoms.

How common is psoriasis?

Psoriasis is common. Estimates are that up to three in every 100 people have psoriasis worldwide, and this number is similar in the UK.

Most people with psoriasis (80-90%) have chronic plaque psoriasis. Other types of psoriasis are rarer. For example:

  • Around 30% of people with psoriasis develop flexural (inverse) psoriasis.
  • Around 8% of people with psoriasis have guttate psoriasis.
  • Around 3% of people with psoriasis get pustular psoriasis.
  • Around 1-2% of people with psoriasis get erythrodermic psoriasis.

Psoriasis risk factors

Risk factors for psoriasis are similar to the causes and triggers of psoriasis. They include:

  • Genetics - having a family history of psoriasis.
  • Smoking.
  • Excessive alcohol.
  • Overweight and obesity.
  • Stress.
  • Certain medications - see triggers.
  • Injury to the skin.
  • Certain infections - for example Strep throat.

How to prevent a psoriasis flare up

Psoriasis flare ups can't always be prevented. However, there are things that make them less likely, such as:

  • Using psoriasis treatment as prescribed.
  • Using moisturisers (emollients) regularly to keep skin healthy, especially in dry and cold weather.
  • Using moisturisers instead of soap to wash with can help to stop skin from drying out.
  • Avoiding, cutting down, or stopping smoking.
  • Avoiding medication that causes psoriasis to flare - ask your doctor if there are any alternative medications that you could use instead. Don't stop regular prescribed medications without speaking to a doctor first, though.
  • Moderating alcohol intake - if you do drink alcohol, avoid drinking over the recommended limits.
  • Controlling stress - relaxation techniques, mindfulness, exercise, and other stress-relieving things can help to control levels of stress.
  • Avoiding itching or scratching the skin can prevent damage which causes psoriasis to spread. Moisturisers are good at relieving symptoms of itch.
  • Avoiding tattoos. Tattoos involve injury to the skin, which can cause psoriasis to develop there. If you do get a tattoo, ensure it's done by a licensed tattoo artist who follows proper procedures to avoid infection.
  • Wearing sunblock when outside, especially on a sunny day.
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