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.
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
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