Tar is a thick, black, oily liquid with a strong and distinctive smell. It is produced by distilling coal or wood. It had been used as a psoriasis treatment and various other skin conditions such as eczema for many years, and has often proved successful.
Why tar should help the skin to heal is a mystery. The pharmaceutical industry has spent huge amounts of money trying to discover just what it is in tar that makes it so effective, but so far without success. Tar contains many thousands of chemical substances and more than one of these may help the skin. It could also be that they have to work in a certain combination in order to have the healing effect, which would obviously complicate matters.
No two batches of tar have exactly the same ingredients and some batches work better than others, so isolating the crucial constituents is, as you can imagine, a difficult task.
Crude coal tar can irritate the skin quite badly so it is rarely used in this form today. Instead you are likely to be given an ointment, cream, gel, shampoo or bath additive that contains a small amount of tar (anything from 0.5 to 5 per cent by weight). These should not irritate your skin, but some of them are messy, smelly and unpleasant to use.
The modem ones tend to be much better in this respect than the old-fashioned kinds. Unfortunately tars have been accused of causing tumors and growths in mice. This has caused some concern but no evidence has been found that they cause such problems in humans.
Sometimes these preparations contain other ingredients as well as tar. For example, they may include some salicylic acid which seems to help banish the crumbly scale on top of a patch of psoriasis. Less commonly, hydrocortisone or other corticosteroids may be mixed in, but there is no proof yet that these creams are any more effective than ones which just contain tar.
How to use them
In the great majority of cases your psoriasis will be mild enough to be treated at home, and you will probably be given a cream or ointment to apply yourself. You will normally be advised to put it on once or twice a day, but this will depend on what it is and you should follow whatever instructions your doctor gives you. Continue with the treatment until the psoriasis has cleared and then stop. It is not a good idea to keep old jars of tar ointment (or for that matter any ointment or cream) with a view to using them again if the psoriasis comes back. The preparations can deteriorate with time so they may not work as well later on. More important, what you think is a recurrence of psoriasis may in fact be something else which will need a different treatment. It is always better to check back with your doctor to make sure.
There is one kind of treatment (called the Goeckermann regime after the American doctor who invented it) which uses tar ointments and baths in combination with ultraviolet light. The tar seems to make people's skins more sensitive to light and so adds to the effectiveness of the ultraviolet therapy. This treatment is more easily carried out in hospital than at home. The sessions have to be repeated daily (or at least five times a week) over a period of two to three weeks, and the tar preparations used tend to be messy. However there are some day treatment centers for psoriasis where you can be given the Goeckermann treatment as an outpatient. At some you can go in the morning for a tar bath, ultraviolet lamp treatment and an application of the ointment and then go off to work as usual.