Chronic plaque psoriasis is the most common form of the condition, accounting for about 90 per cent of cases. It is characterised by well-defined, thickened, red patches of skin covered with silvery scales that are readily shed. The most commonly affected areas are the scalp, the extensor (outside) surfaces of the limbs (e.g. shins and elbows) and the lower back.

The plaques can crack and bleed and many find psoriasis patches itchy. The scalp is affected in about 80 per cent of psoriasis patients, and profuse shedding of skin scales from the scalp can be a serious problem for some people. Psoriasis can also affect the flexures (e.g. armpits, groin, genitals).