Psoriasis is a chronic and distressing skin disease that afflicts about two million people in the US, and a further 1.12 million in the UK alone. It is most commonly found in the US and northern Europe, affecting, in all, about 2 per cent of every population in those countries.

One insight into the condition can be drawn from a social study of the Australian Aboriginals, who don't suffer from the condition while living in the bush, but develop it when moved to the city.

Another study, of the Greenland Eskimos, has led to the introduction of fish oils into the vast armament of treatments used by medicine. The study discovered a low incidence of psoriasis among the Eskimos, which has been attributed to the mainly fish diet, high in omega 3 fatty acids. As a result, clinical trials discovered that fish oils were helping the condition (Geraldine McCarthy, Medical College of Wisconsin, The Lancet, 28 September, 1991).

However, if you do not live in the bush or an igloo, the chances are you will be treated by your family doctor as dermatologists in the UK seem a rare breed. Equally possibly, you may be treated with the great new wonder drug calcipotriol (Dovonex), especially after it received a glowing report following a major test by Dr K Kragballe et al of the Marselisborg Hospital, Aarhus, Denmark (The Lancet, 26 January 1991).

It is but one of 15 possible conventional treatments available to doctors (although, at the time of going to press, WDDTY was still counting). The chosen treatment will depend on the condition, and also your doctor, who may prefer white soft paraffin to drugs. As two dermatologists from the Royal Hallamshire Hospital put it, in writing in The Lancet (31 August 1991): "The diversity of current therapies for psoriasis reflects the fact that many existing treatments are lacking efficacy, convenience of use, or freedom from adverse effects."

Other conventional treatments (see box, p 2) include mild steroid creams, ultraviolet light B (not to be confused with UVA suntan machines) and zinc. Most alarming of all are cyclosporin and methotrexate (Maxtrex). Cyclosporin is a powerful immunosuppressant, extensively used in organ transplants, while methotrexate, used since 1955 to treat psorisasis, was initially intended to treat cancer.

Calcipotriol owes much its ascendancy to the fact that it is better than the devil you know. That devil is dithranol (Cignolin in Europe and Anthralin in the US) which has been used for 65 years to treat the condition. It is applied directly on the plaques and lesions, provided they are not on the scalp, but has side effects that can only further sap the confidence of the sufferer. The cream causes a brownish staining of the skin, and bandages have to be worn as it can come off on to clothes and bedding. The stains cannot be washed out.

Calcipotriol, by comparison, is colourless and invisible on the skin, and it does not stain. However, it should be used only on certain types of people and for short periods, and long term side effects are not known, although researchers worry about its effect on the calcium levels of patients.

Amid the stampede of enthusiasm for this drug, objections raised by some doctors and researchers about study designs and proper testing has been ignored. Drs C Long and R Mack from the Department of Dermatology at the University of Wales College of Medicine wrote in to The Lancet (13 April 1991) to complain about the design of Kragballe's famous study. Long and Marks argue that the study compared calcipotriol with steroids, which "is by no means the best treatment for psoriasis". Because they didn't compare like with like or the most effective treatment (or even a non medical control ointment) it is "difficult to know how much therapeutic activity is attributable to the drug and how much to the vehicle" ie, the cream itself.