Analogs of vitamin D3 metabolites (not the nutritional vitamin itself) can be used to treat psoriasis. Probably the best-known analog is a drug called calcipotriol, which is sold under the brand name Dovonex®. The drugs work primarily by slowing the rate at which psoriatic skin cells mutliply, and by making newly-formed skin cells differentiate into their proper forms. How, precisely, they accomplish this is not yet fully known.
These drugs are usually well-tolerated, but the most-common side effect is skin irritation at the sites of application (these are topical drugs, available as ointments, creams, or scalp solution in the case of Dovonex®). Some studies have found that up to 20% of patients experience these typically mild side effects. The risk and severity of side effects can be dramatically increased by occluding (or covering) the drug, and so Dovonex® isn’t generally recommended for “skin fold” areas, where skin can occlude other skin (such as the groin or armpits). Dovonex® is also not generally recommended for use on the face, where the skin is particularly sensitive.