Coal Tar Products for Psoriasis

Ancient Remedy - New Worries?

Long before steroids, there was coal tar. In fact, coal tar has been used in the treatment of skin diseases for over a century. It is the left over by-product of coal processing and distillation. There are thousands of compounds in coal tar, and only a fraction of these are identified. For this reason, it is unlikely that coal tar would be approved by the FDA if presented for new drug status. In any case, coal tar is grandfathered in -- it has been a medication longer than the FDA has existed. Before corticosteroids were first recognized or synthesized for use in inflammatory skin diseases, coal tar was there to help ease the itch. Some of my older professors originally practiced in the pre-corticosteroid era and were very adept at the use of coal tar compounds for skin therapy. Coal tar preparations appear to exert their anti-psoriasis benefits by interfering with DNA and thus slowing down skin cell growth and turnover. The long-term result is thinning of the psoriatic plaques.

How is Coal Tar Used?

Coal tar is found in dozens of over-the-counter psoriasis and dandruff shampoos, as well as creams, gels and bath additives. Compound pharmacists can mix crude coal tar (a black, thick paste) or coal tar solution (an 20% alcohol-based liquid) with all sorts of bases, including steroid creams and ointments. One effective remedy for hand and foot psoriasis is a compound of a steroid with 5% coal tar solution and 2% salicylic acid which a pharmacist can mix up with a prescription from your doctor.

Coal tar is often used in combination with phototherapy, as it sensitizes the skin to ultraviolet radiation. Care should be taken in avoiding excess sun exposure when using coal tar shampoos and other preparations.

Is Coal Tar Dangerous?

With many unknown ingredients, the question is not too easy to answer definitively. However, 5% or greater coal tar is classified as a carcinogen (cancer-causing agent) by the World Health Organization's International Agency for Research on Cancer. In this regard, it is in the same category as methoxsalen (used in PUVA therapy for psoriasis) and solar radiation, two other forms of psoriasis treatment. Alcoholic beverages and tobacco also qualify in this category.

There are not too many over-the-counter products that carry the full 5% crude coal tar concentration considered carcinogenic by the WHO, however California law is much more strict in this regard and considers even 0.5 coal tar dangerous enough to require a warning on a product's label . The FDA, in contrast, considers 0.5 to 5% OTC coal tar preparations safe for psoriasis, and there is really no evidence linking these weaker preparations to an increased risk of cancer. Although coal tar compounds have been found in the urine of users of an experimental tar-based shampoo, the concentration of coal tar used was around 100 times greater than that of common OTC shampoos.

Still Useful in Many Regards

For the most part, worries about coal tar are probably overblown. It has several merits, including very low cost and absence of steroids (and, therfore, steroid-related side effects). Typically it is left on for about two hours and rinsed off. This short contact type of therapy helps to prevent staining of clothing and fabrics often seen with use of coal tar.

_Sources:

van Shooten F-J et al. Dermal uptake of polycyclic aromatic hydrocarbons after hairwash with coal-tar shampoo.Lancet 1994 344 1505-1506.

Lowe NJ. Psoriasis Therapy: A Current Perspective. West J Med 1983;139:184-9