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Coal tar and Dithranol
General information
Coal tar (also known as liquor carbonis) is a black, viscous liquid by-product obtained during the distillation of bituminous coal and as a by-product in the manufacture of coke. It is used in cosmetic products as a biocide and denaturant and in antidandruff shampoos. It is used therapeutically to manage psoriasis and atopic dermatitis [ ]. Crude coal tar is composed of 48% hydrocarbons, 42% carbon, and 10% water.
Adverse reactions to coal tar have been reviewed [ ]. There have been reports of contact sensitivity, with phototoxicity due to photosensitization. In women exposed to coal tar during pregnancy there was no evidence of increased risks of spontaneous abortion or congenital disorders. Cancer epidemiology studies have not shown an increased risk of cancers.
Dithranol (1,8-dihydroxy-9,10-dihydroanthracen-9-one, also known as anthralin or cignolin), which was originally introduced as an alternative to a similar compound, chrysarobin (methyltrioxyanthracene), is a coal tar derivative, used for the same therapeutic purposes [ ]. Like coal tar, it stains the skin yellowy-brown and can cause local sensations of burning and irritation.
This monograph deal with adverse reactions that arise during the therapeutic use of coal tar and its derivatives, not environmental exposure.
Drug studies
Comparative studies
Once daily dithranol cream 0.25–2% for 30 minutes (n = 54) has been compared with twice-daily calcitriol ointment 3 μg/g (n = 60) in an 8-week randomized, open, parallel-group study in 114 patients with plaque psoriasis [ ]. Skin irritation was reported by 39% of those who used dithranol compared with three patients who used calcitriol. Three patients who used calcitriol and four who used dithranol reported adverse skin reactions (pruritus, erythema, rash, dry skin, eczema).
In 80 patients with alopecia areata who were treated with topical glucocorticoids, tretinoin 0.05%, dithranol paste 0.25%, or white soft petrolatum jelly, dithranol was associated with dermatitis and hyperpigmentation [ ].
Systematic reviews
The evidence base for the therapeutic efficacy of coal tar products in the treatment of psoriasis and atopic dermatitis is poor. In a systematic review of 25 studies, only two were placebo-controlled [ ]. Staining and odor were the main deterrents to the use of coal tar products.
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