Coal tar is a thick black liquid made by heating coal at very high temperatures under anaerobic conditions. Coal tar has a long history for topical treatment of skin conditions, including psoriasis and atopic dermatitis.
+ May have anti-inflammatory effects
+ May enhance shedding of dead skin cells
- These preparations are messy to use, can stain clothes, and have an unpleasant odor
- Coal tar may also cause skin irritation and increase photosensitivity to ultraviolet light
Coal tar comes in different concentrations in various formulations, and contains a huge 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 proposed actions 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, that is unusually high in psoriatic plaques.
A large number of studies support the use of coal tar
In a recent review it was concluded that most studies (21 of 25) supported the use of coal tar products in psoriasis or atopic dermatitis, but also that the level of evidence is not strong . A large number of studies expected to provide further information on this are ongoing.