Michael Traub, ND, is an alternative dermatology expert.

Getty ImagesQ: What do I need to know about the risks of tar-based shampoos and other treatments?

A: Coal tar has been a dermatological treatment for over 150 years, both by itself and in combination with other treatments. It is generally lacking in serious side-effects. The biggest complaints with coal tar are typically the smell (just like the tar-covered pilings of a pier), and messiness (its typically delivered in an ointment base).

A major concern of many people considering coal tar as a psoriasis therapy is cancer. While occupational exposure to coal tar (miners, asphalt workers, or chimneysweeps for example) may be responsible for some lung, skin, and scrotal cancers, no relationship between even high therapeutic dosages of coal tar for psoriasis and any form of cancer has been established. Some 25-year-long studies have found no increase in any form of cancer over what would be expected without coal tar.

A problem that many people are not aware of, though, is that coal tar is a photosensitizer. After applying coal tar to the skin, it is important to be more careful than normal when exposing the same skin to sunlight or other UVB light sources. The photosensitivity can make tanning and burning much more easy, and skin damage can worsen psoriasis.

Typical therapeutic concentrations of coal tar are around 10% coal tar solution, which is equivalent to 2% crude coal tar.