Coal tar

Being one of the oldest treatments for psoriasis, it remains a moderately effective option; however it is rarely popular with patients. Products tend to have a tar smell associated with them, although the weaker strengths have a less potent aroma. The main potential side effects associated with coal tar include skin irritation and folliculitis. It is thought that coal tar works by reducing epidermal thickness and by suppressing epidermal DNA synthesis. It is a complex substance containing some 10,000 different compounds of which only 50% have been identified (Camisa, 2004b). Some researchers are working on identifying which compounds within coal tar are actively anti-psoriatic in the hope that by distilling these out a more acceptable and yet effective treatment may become available (Arbiser et al., 2006).

A number of studies have been carried out comparing the efficacy of calcipotriol and coal tar. The systematic review carried out in 2000 confirmed that calcipotriol was more effective (Ashcroft et al., 2000). Like calcipotriol, coal tar can be used in combination with UVB; this is known as the Goeckerman regime named after the doctor who first published the use of this methodology in 1925. Coal tar enhances sensitivity to the UVB and may be UVB sparing. Tar and UVB therapy offered within a health care facility (day care) continues to provide treatment that over a period of time improves treatmentresistant psoriasis. In a small study, 100% of patients had 75% improvement in their PASI at the end of a 12-week treatment programme using the Goeckerman regime with narrowband UVB (Lee and Koo, 2005).

There have been some concerns about the carcinogenicity of coal tar. There is no doubt that coal tar does contain some carcinogenic substances as has been demonstrated by animal and occupational studies. There is, however, no research within the dermatological field of the use of coal tar preparations and whether these increase the risk of internal skin tumours (Roelofzen et al., 2007).

Method of application
On a day-to-day basis in primary care, weak coal tar solutions are useful for widespread small plaques of psoriasis as treatment does not need to be accurately applied to the plaques only. They can be applied 2–3 times per day. These weaker solutions will sink into the skin and a patient can get dressed after this. However, there is always a risk of some staining and it may be easier for patients to apply the treatment at a point in time when they can wear ‘messy’ clothes afterwards. There is no evidence to support the amount that should be used; however enough to cover the areas comfortably is a sensible suggestion. Coal tar preparations should be applied following the lie of the hair in order to reduce the likelihood of folliculitis.
In a health care facility, where stronger strengths of tar may be used, tubular netting bandages can be applied afterwards to keep the tar close to the skin. It is impractical to get dressed during this kind of treatment; some units are therefore advocating ‘short contact’ coal tar. Where it is left on for 4–6 hours and then washed off and an emollient applied. This is particularly useful in a day-care setting where a patient can come in for a set period of time and then leave.

Tar is particularly helpful for the management of scalp psoriasis. In combination with coconut oil and salicylic acid, it can be successfully used to descale thickened plaques. Tar-based shampoos may be used in conjunction with other treatments or in less severe scalp psoriasis may be sufficient to keep the condition under control without the need for other treatments. Table 8.3 outlines how scalp treatments should be applied.