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Topical Coal Tar

Coal tar has been used to treat psoriasis over many years. Although often considered to be safe, there have been doubts about its safety since the 1940's, and recent evidence has provided further evidence on this. There are several commercially available creams, which contain between 0.4% and 2% crude coal tar, and also shampoos, which have a coal tar content of up to 2.5%. Crude extracts of coal tar can be made up in white or yellow soft paraffin, or emulsifying ointment. Coal tar solution is already diluted, so the true concentration of tar in commercial tar products is lower than stated and not equivalent to crude coal tar.

Efficacy

Coal tar is an effective treatment for inducing remission in psoriasis. Coal tar preparations of between 1 and 5% in white or yellow soft paraffin are as effective as higher concentrations. The use of higher concentrations, which has been traditionally advocated, has no evidence-based foundation and is best avoided, especially as it restricts outpatient use.

Safety, side-effects and patient acceptability

Coal tar preparations smell. The crude extract preparations smell more and are messier to use. Recently there has been renewal of concern about the potential carcinogenicity of coal tar products. Occupational exposure to coal tar is associated with an increased risk of skin cancer, and some studies have shown that skin cancers are more common in patients with psoriasis, although not all studies were controlled for the confounding effects of smoking and alcohol consumption, and many squamous cell carcinomas are accounted for by the effects of photochemotherapy, which may mask other effects. Experimental studies have shown that the use of coal tar shampoos results in the absorbtion of appreciable amounts of polycyclic aromatic hydrocarbons (PAH), substances identified as being carcinogenic. In view of this, the Dutch delegation to the European Commission has suggested limiting the concentration of benzo[a]pyrene (one of the carcinogens known to be in coal tar) in commercially available coal tar products. In Germany, cosmetic manufacturers have voluntarily agreed to ban coal tar from their shampoos.

Despite the above,there is at present, no firm epidemiological evidence that topical tar products cause cutaneous or internal cancer. The extent of percutaneous absorption of tar derivatives in tar-treated patients with psoriasis is currently unknown. It is considered reasonable , therefore, for topical tar containing products remain available.

Synergy with other treatments

Tar treatments act synergistically with ultraviolet B radiation in the traditional Goeckerman regimen. This can be a very effective method of clearing mild psoriasis. In addition, there are reports suggesting that tar and topical steroids have a synergistic effect, and some dermatologists adopt a regimen of a moderately potent topical steroid by day, with tar by night, for ease of patient use. A modified Goeckerman regime using narrow band UVB as the adjunct achieved PASI 75 (75% improvement or more in PASI score) in 95% of cases.

References

Stern RS, Laird N. The carcinogenic risk of treatments for severe psoriasis. Cancer 1994; 73: 2759-64

Van Schooten FJ, Moonen EJC, Rhijnsburger E, et al. Dermal uptake of polycyclic aromatic hydrocarbons after hairwash with coal-tar shampoo. Lancet 1995; 344: 1505-6

Zackheim HS. Should coal tar products carry cancer warnings? Cutis. 2004 May;73(5):333-4.

Lee E, Koo J. Modern modified 'ultra' Goeckerman therapy: a PASI assessment of a very effective therapy for psoriasis resistant to both prebiologic and biologic therapies. J Dermatolog Treat. 2005 Apr;16(2):102-7.


This information forms part of the current BAD guidance document for the general management of psoriasis.  Other sections in the document comprise: