Coal tar is a thick black liquid made by heating coal at very high temperatures under anaerobic conditions. Coal tar has a long history as a topical treatment of skin conditions, including psoriasis. Coal tar comes in different concentrations in various formulations, and contains a large number of different substances. Some of these are thought to have anti-inflammatory properties, which could be helpful in for instance scalp and plaque psoriasis. Other suggested benefits include enhanced shedding of the dead cells in the upper skin layer, that helps reduce plaque thickness, and slowing down the growth of skin cells, which is unusually high in psoriatic plaques.
In a review from 2010 it was concluded that most studies (21 of 25) supported the use of coal tar products in psoriasis, but also that the level of evidence is not strong1. A large number of studies expected to provide further information on this are ongoing. In a study in people with psoriasis where a 5% coal tar lotion was used three times daily for 12 weeks it was shown that 27% of the patients achieved a clearance or marked improvement2.
It is important to note that the safety profile of 5% coal tar lotion did result in treatment-related side effects being mild to moderate in severity.
The concentration of an ingredient is important for its efficacy, and therefore products containing the same ingredient may not necessarily have the same effect as in the studies mentioned above.
Updated: July 2017