Photodynamic therapy (PDT) is a two-stage treatment that combines light energy with a drug (photosensitizer) designed to destroy cancerous and precancerous cells after light activation. Photosensitizers are activated by a specific wavelength of light energy, usually from a laser. Photodynamic therapy treats premalignant growths by using special drugs called photosensitizing agents, along with light, to kill pre-cancerous cells. The drugs only work after being activated by certain wavelengths of light. The process also is known as PDT, photo radiation therapy, phototherapy and photo chemotherapy. Most patients are able to their normal activities 24-48 hours after photodynamic therapy. Avoiding direct sunlight for the first 1 to 2 days in crucial to avoiding getting an exuberant (red) response.