Oxsoralen is a brand name for methoxsalen, a substance which occurs naturally and is prescribed in capsules for the treatment of severe, recalcitrant, disabling psoriasis. An hour to an hour and a half after taking the Oxsoralen capsule, you are then exposed to long wave ultraviolet radiation. You may be exposed to differing quantities of ultraviolent light depending on whether your skin always burns and never tans, always burns but sometimes tans, sometimes burns but always tans, or never burns but always tans.