Phototherapy is the controlled exposure to sunlight or artificially produced ultraviolet radiation for medical treatment. Measured doses of specific ultraviolet wavelengths are used to treat many skin conditions, including dermatitis, psoriasis and vitiligo. Ultraviolet (UV) light may be used alone or in combination with topical or oral medications.

The ultraviolet spectrum is divided into ultraviolet A (UVA) and ultraviolet B (UVB) light. Topical moisturizers or coal tar preparations may be used to enhance UVB effects. UVA may be used with drugs called psoralens to produce results that ultraviolet light or the medication alone could not accomplish. Psoralens may be administered orally or applied topically through creams, ointments, lotions or a bath water solution.

The doses of ultraviolet light used are often determined by testing skin that is not usually exposed to sunlight (e.g., lower back, buttocks). This allows a dermatologist to evaluate the patient’s individual sensitivity to ultraviolet light. Moisturizers or medications used with phototherapy are usually administered before ultraviolet light exposure.

Ultraviolet light is typically administered in doses that cause the skin to just barely turn red. With repeated exposures, a gradual increase of the dose is required. The rate of increase depends on whether or not psoralens are used, treatment frequency and the outcome of the previous treatment. Phototherapy treatment is generally continued until complete remission is achieved or no further improvements can be obtained. Maintenance sessions may be used to increase remission times.

Although phototherapy can greatly improve or even clear many skin conditions, it poses risks. Short-term risks include burning similar to sunburn that can be very severe. Long-term risks include photoaging of the skin and the formation of skin cancer. Oral psoralens pose further risks, such as the risk of cataract formation (condition in which the lens of the eye becomes cloudy).

It is important to have the skin checked for skin cancer, and other negative conditions that may develop due to phototherapy, for many years after the treatment has ended.