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Retinoids are a derivative of vitamin A and can be used both orally and by direct application to the skin. They should not be confused with the vitamin A products that are bought from chemists or supermarkets as vitamin supplements.

Oral acitretin is an effective vitamin A based treatment for psoriasis. It is only prescribable by hospitals. It is used by dermatologists in hospital practice for severe, extensive, refractory psoriasis. It has a long half-life which means it remains in the body for a considerable period of time after treatment and its persistence in tissues requires that contraceptive measures must be used by women during and for at least 2 years following therapy as it can harm the development of unborn children.

By contrast, the topical retinoid, tazarotene is used for the treatment of mild to moderate plaque psoriasis. It binds to nuclear retinoic acid receptors in the skin promoting normal differentiation of skin cells and reducing the formation of patches of raised skin.

It also reduces the formation of cytokines and interleukins, 2 chemicals in the body that are responsible for causing inflammation. In other words it acts by reducing the inflammation AND reducing the rate that the skin cells develop plaques.

Tazarotene's specificity and its low systemic absorption means that it produces fewer side effects than the earlier retinoids. It has a half-life of only 18 hours and does not accumulate on repeated administration.

Contraceptive measures are only necessary for women during treatment.