Psoriasis explained: types, causes, diagnosis and treatment categories

Thick, scaly patches that do not go away and are not infectious can be unsettling, especially when someone tells you they are "just dry skin" and the patches keep returning. Psoriasis is one of the more common explanations for that pattern. This page sets out what psoriasis is, the types doctors describe, who tends to develop it, how it relates to joint disease, what sources say about triggers and diagnosis, the main treatment categories, and the conditions it is linked with. It is educational and does not replace an assessment of your own skin.

What psoriasis is

The National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS) describes psoriasis as a chronic disease in which the immune system becomes overactive, causing skin cells to multiply too quickly. Patches of skin become scaly and inflamed. NIAMS says the causes involve genetics and environmental factors.

"Immune-mediated" is the phrase you will see in clinical writing. It means the inflammation is driven by the body's own immune signals, even though no germ is the cause. The NHS puts the same idea more cautiously: the skin produces more skin cells than usual and forms patches, the exact reason is not known, and an immune system problem is thought to be involved.

Psoriasis is not contagious. You cannot catch it from touching, sharing a pool or living with someone who has it. The NHS also notes that it cannot be cured, but treatments can manage symptoms, which often flare and then improve.

Plaque and the other types

Doctors describe several forms. NIAMS lists five. Plaque psoriasis is the most common, with raised red patches covered by silvery-white scales. Guttate psoriasis shows as small red dots, often in children or young adults, frequently after an upper respiratory infection. Pustular psoriasis brings pus-filled bumps, usually on the hands and feet. Inverse psoriasis appears as smooth red patches in skin folds. Erythrodermic psoriasis is rare but serious, with red, scaly skin over most of the body.

Colour matters in what you see. The NHS explains that patches look pink or red on white skin and grey on brown or black skin, and that psoriasis can appear on the scalp, in skin folds and in the fingernails. Patches are usually 1 to 2 cm across, according to the NHS, though they can join into larger areas. Because inverse and pustular forms look quite different from plaques, photographs online can mislead, and a clinician is the right person to say which pattern, if any, applies.

Who gets it and how common it is

NIAMS says anyone can get psoriasis, that it is more common in adults than in children, and that it affects men and women equally. The NHS adds that symptoms often start between ages 15 and 25, or between 50 and 60. A family history raises the risk.

The scale is large. The World Health Organization's 2016 global report on psoriasis, summarised in a WHO news release, states that psoriasis affects approximately 100 million people worldwide. The report also states that there is no known cause or cure. For the United States, the National Psoriasis Foundation states that more than 8 million Americans have psoriasis, citing a 2021 study in JAMA Dermatology, and that about 125 million people worldwide, or 2 to 3 percent of the population, are affected.

Notice the gap between the WHO and foundation world totals (100 million versus 125 million). They come from different compilations of national studies and different years. Figures like these are best read as estimates in a range.

Psoriatic arthritis

Psoriasis can affect more than skin. Psoriatic arthritis is an inflammatory arthritis that NIAMS says causes pain, swelling and stiffness in the joints and at the points where tendons and ligaments attach to bone. Signs include stiff, swollen or painful joints, neck or back pain, and heel pain at the Achilles tendon. NIAMS warns that untreated it can cause irreversible joint damage, which is why it advises prompt medical attention.

How many people with psoriasis develop it is less certain than you might expect. The WHO report states that between 1.3 percent and 34.7 percent of people with psoriasis develop psoriatic arthritis, a wide range that reflects differences in study populations and how the condition was identified. The National Psoriasis Foundation cites an estimate of about 30 percent, based on a 2013 study of patients seen in dermatology clinics in Europe and North America. The NHS says some people develop it as they get older and suggests seeing a doctor if joint pain, stiffness or swelling keeps coming back.

Nails can offer a clue. The clinical reference DermNet reports that nail psoriasis may raise the risk of developing psoriatic arthritis. The overlap between skin, nails and joints is one reason a dermatologist may ask about joint symptoms, and hair and nail disorders describes nail changes in more detail.

Triggers and risk factors

A trigger is something associated with starting the condition or setting off a flare. NIAMS lists infections (especially streptococcal infections and HIV), certain medicines (such as some used for heart disease, malaria or mental health problems), stress, skin injuries, smoking and obesity. It also notes that a bad sunburn can trigger erythrodermic psoriasis.

The NHS list is similar and adds alcohol, pregnancy and, for some people, sunlight. That last point can seem odd, because NIAMS also suggests small amounts of sunlight for living with psoriasis while advising people to ask their doctor, since too much can worsen it. The pattern is not the same for everyone.

A short caution applies. Lists of triggers describe associations seen in groups of people. They do not mean that avoiding one will clear your skin, nor that a flare was your fault. If you suspect a medicine is involved, the decision to continue or change it belongs to the prescriber who knows your whole history.

How psoriasis is diagnosed

Most of the time the diagnosis rests on looking and asking. NIAMS says a doctor examines the skin, scalp and nails, and asks about itching or burning, recent illness or stress, medications, family history and joint tenderness. A small skin sample (a biopsy) may be taken to examine under a microscope when the picture could be another condition. The aim is to rule out look-alike skin conditions.

For a wider view of the process, see how dermatologists diagnose skin conditions, and skin biopsy, what to expect for what a biopsy involves. Preparing for a dermatology visit lists things that help a clinician, such as a timeline of symptoms and a record of medicines.

Treatment categories

There is no cure, but the sources agree that treatment can control symptoms. NIAMS says the doctor's choice depends on the psoriasis type, severity, body location and side effects. The categories it names are listed here to show how the field is organised; none is a recommendation.

• Topical treatments: creams, ointments, lotions, foams or solutions applied to the skin. NIAMS mentions corticosteroids, vitamin D-based products, retinoids, coal tar, anthralin and newer topical agents, generally for mild to moderate disease. The NHS also lists emollients (moisturisers).
• Light therapy (phototherapy): ultraviolet light given in a doctor's office or, in some cases, with a prescribed home unit, usually when large areas of skin are affected.
• Systemic medicines: oral or injected drugs that act throughout the body, usually for severe disease, which NIAMS says include methotrexate, oral retinoids, immunosuppressants and newer oral agents.
• Biologics: injected medicines that block specific immune molecules to reduce inflammation.

Combinations are common, according to the NHS, which also says a GP can prescribe some treatments or refer a person to a dermatologist. Which category suits a person depends on the type, severity and location of the psoriasis, and on side effects, as NIAMS says. That is a question for a specialist.

Associated conditions

Psoriasis is increasingly understood as a condition that can accompany others. NIAMS lists psoriatic arthritis, cardiovascular events such as heart attacks and strokes, and mental health problems such as low self-esteem, anxiety and depression. It also names certain cancers, Crohn's disease, diabetes, metabolic syndrome, obesity, osteoporosis, uveitis (inflammation inside the eye), liver disease and kidney disease.

The National Psoriasis Foundation similarly states that people with psoriasis have higher rates of cardiovascular disease, stroke, metabolic syndrome and depression. The NHS points to metabolic syndrome, which can lead to diabetes and cardiovascular disease, and to inflammatory bowel disease such as Crohn's disease. The WHO report likewise lists arthritis, cardiovascular disease, metabolic syndrome, inflammatory bowel disease and depression.

What does that mean for an individual? It means association, not destiny. Most people with psoriasis will not develop every condition on those lists, and the reasons for the links (shared inflammation, shared risk factors such as obesity and smoking, or both) are still being worked out. The practical consequence sources describe is that clinicians may look at general health, not only the skin.

Living with psoriasis

Day-to-day, NIAMS describes habits that may help: moisturising well, bathing in lukewarm water with a mild soap, applying a heavy moisturiser while the skin is damp, keeping a healthy weight, not smoking, limiting alcohol, and identifying and avoiding personal flare triggers such as stress, cold weather and skin injuries. It also notes that the Mediterranean diet has shown benefits in studies. These are general statements from a public body, and they sit alongside medical treatment, not instead of it.

The visible nature of psoriasis can be hard. The WHO report highlights discrimination and stigma associated with the disease, and NIAMS notes an increased risk of anxiety and depression and suggests support groups or a mental health provider. The NHS likewise says mental health support is available if psoriasis affects wellbeing. For related inflammatory skin conditions, eczema and atopic dermatitis is a useful comparison.

Honest limits

Much remains unsettled. The precise causes are not fully known. Prevalence figures vary by source and method, and the proportion who develop psoriatic arthritis is reported anywhere from about 1 percent to about 35 percent depending on the study. Treatment advice changes as new medicines arrive, and public bodies in different countries organise access differently.

This page is general education, has not been medically reviewed, and cannot say what applies to you. Individual care varies, and your own doctor or specialist has the final word.

The short version

Psoriasis is a common, chronic, non-contagious condition in which an overactive immune system speeds up skin cell growth. It comes in several forms, can involve the joints and nails, and is linked with a range of other conditions. Doctors usually diagnose it by examination, and treatment ranges from skin applications and light therapy to systemic and biologic medicines, chosen by a clinician for the individual.