Coal tar, also known as liquor carbonis detergens, is a thick, black sticky material occurring as a by-product in the industrial use of coal for fuel.1 A naturally occurring phenomenon of tar is found in the famed La Brea Tar Pits in Los Angeles, California. Coal tar products are most often used topically for a variety of skin conditions, including psoriasis. Coal tar is one of the oldest known treatments for psoriasis, as it reduces scaling, itching, and inflammation. Its exact working mechanism is not known.

Background

Coal tar was supposedly discovered in 1665, but it is discussed in many cultural writings as being used for medicinal purposes centuries earlier.2 Today, it is listed in the World Health Organization’s “List of Essential Medications.”3 Coal tar products are most commonly used for psoriasis, an auto-immune inflammatory condition primarily of the skin that affects an estimated 125 million people around the world.4 

Science 

Coal tar products’ mechanism of action for this condition is felt to involve suppression of specific deoxyribonucleic acid (DNA) synthesis, thereby inhibiting keratinocyte proliferation that results in the thick, plaque formations.5 In addition to blocking the formation of these plaques, coal tar breaks down keratin layers that are already formed.6 There also may be direct anti-inflammatory and antimicrobial action.4 

Further studies have examined the levels of helper T-cells and interleukin-12 (IL-12) in patients with psoriasis and found significant differences in pre- and post-treatment levels. IL-12 is a recognized pro-inflammatory cytokine that is consistently elevated in psoriasis patients.7 IL-12 appears to suppress the production of specific T-cell production. In one study of 27 psoriasis patients, the level of these beneficial T-cells was significantly higher in patients after a prescribed topical coal tar treatment known as the Goeckerman Regimen.7 A similar trial of 55 psoriasis patients measured pre- and post-treatment serum levels of IL-12, with serum levels of IL-12 lower after Goeckerman therapy than before treatment, potentially confirming the anti-inflammatory effect of this coal tar therapy.8 

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More recently, increased attention has been given to the use of the Goeckerman therapy in cases of severe psoriasis that have proven resistant to biologic therapy. The Goeckerman Regimen typically consists of a pre-treatment of topical corticosteroid creams or liquids in the most severe cases. Then, patients are treated with a regimen of total-body coverage of a coal tar preparation for 4 to 5 hours daily for up to 6 weeks.9 Ultraviolet B phototherapy treatments are prescribed because coal tar is a photosensitizer that can increase the effectiveness of traditional phototherapy.9 In a retrospective review, researchers at the University of California at San Francisco examined records of 53 patients with severe plaque psoriasis who had been treated with various biologic therapies without success.10 These patients were then treated with either add-on therapy with the Goeckerman Regimen or the Goeckerman Regimen alone. The results of this review showed that 40 patients (74%) experienced > 80% skin clearing of the condition.10 

Additionally, there is evidence to suggest that topical coal tar may help reduce the bothersome pruritus associated with psoriasis.11 The proposed mechanism of this amelioration is the interference of the C-fiber nerve transmission of substance P. Substance P is a generalized cytokine that is strongly associated with any anti-inflammatory condition, especially that of the skin.

This article originally appeared on Clinical Advisor