Dithranol has been used for more than 100 years in the treatment of psoriasis. It is a chemical of plant origin, taken from the bark of the South American araroba tree.
Dithranol in Lassar's paste (a salicylic acid and zinc oxide paste developed by German dermatologist Oskar Lassar) is used most successfully for hospital inpatients. The use of dithranol is not without drawbacks: it can stain both skin and clothing and can burn non-affected skin if its application is not precise.
So, careful application in hospital is far safer, whereas home use is often less effective because of the difficulties in application. There are also a large number of concentrations available to doctors and the tendency has been to prescribe people different concentrations to be used in a step-up fashion, whereby the strength of the product is increased based on the level of benefit or adverse reaction.
Proprietary products containing dithranol are more acceptable. They can be applied to the psoriatic plaques and left for up to one hour before washing off. This method reduces the risk of dithranol burning and staining but it’s still vital that you take great care when applying the product. It’s also advisable to sit on an old towel to protect fabrics where necessary.