It has been known for a long time that coal tar (2-5%) helps psoriasis and it is available as crude coal tar coal, tar lotion, and in refined forms incorporated into ready made creams, lotions and shampoos. In general, the more messy and smelly the tar preparation is, the better it is likely to work - so please be prepared to persevere with it.
A chemical similar to those found in tar may be used on its own - known as Dithranol or Anthralin. This must be used cautiously as it can irritate, but a strength and base can usually be found to suit and help the individual person's psoriasis. Tar treatments can lead to steady and effective control. It is usually best to apply a tar or drithanol preparation daily, to be followed later by ultra-violet light treatment.