Chronic plaque psoriasis is the most common type of psoriasis. Although any part of the body may be affected, the most commonly affected sites are the elbows, knees and scalp. 'Topical' treatments (i.e. treatments applied to the skin) are usually tried first. These include vitamin D products, topical corticosteroids, tar-based preparations, dithranol, salicylic acid and vitamin A products. As chronic plaque psoriasis is a long-term condition, it is important to find out which treatments work best and what adverse-effects they have.
The evidence was based on 131 studies that included 21,448 people. Studies were typically about 6 weeks long, but this ranged from 1 to 24 weeks. Vitamin D products were found to work better than placebo (the base cream or ointment). Potent (strong, e.g. betamethasone dipropionate) and very potent (very strong, e.g. clobetasol propionate) topical corticosteroids were also effective. Dithranol and tazarotene also worked better than placebo. Their effects were similar to vitamin D products.
Some studies compared vitamin D products directly with potent or very potent corticosteroids. These products had similar effects when applied to the body, but corticosteroids appeared to work better for scalp psoriasis. However, treatment that combined vitamin D with a potent corticosteroid was more effective than either vitamin D alone or topical potent corticosteroid alone. Vitamin D products performed better than coal tar, but studies found different results when comparing vitamin D with dithranol. Vitamin D products were more effective when covered (occlusion), or when applied twice-daily rather than once-daily.
Potent corticosteroids were less likely than vitamin D to cause 'local adverse events', such as skin irritation, and people were therefore more likely to stop using vitamin D products. Tazarotene was more likely to cause local adverse events than placebo, and people with psoriasis were therefore more likely to stop using it. When studies examined whether topical treatments had effects within the body ('systemic adverse events'), we found no difference between placebo and any other treatment. However, this may be because many trials did not properly assess systemic adverse events rather than because there really was no difference.
There are very few long-term studies that can help doctors and people with psoriasis decide on the best way to treat this chronic condition.