This medication is used to treat cutaneous T-cell lymphoma (CTCL), a type of cancer that affects the skin and blood and sometimes the lymph nodes and other organs. CTCL is caused by the uncontrolled growth of abnormal white blood cells in the skin. This drug is used in a procedure called photopheresis. Some of your blood is removed from your body through a vein and goes into a special machine that separates the white blood cells. The machine adds methoxsalen to these white blood cells, then shines ultraviolet (UV) light on them. Then the machine returns the treated cells (and the rest of your blood) to your body through the same vein. Your immune system is thought to react to the treated cells and other similar untreated T-cells that are not working properly. This effect helps to restore your immune balance and lessens the skin problems (such as rash, plaques, tumors) of CTCL. Methoxsalen is known as a psoralen photosensitizer. It works by making the treated white blood cells more sensitive to UV light.