For years now, doctors have used ultraviolet lamps to treat psoriasis and to kill airborne tuberculosis bacteria, and lasers have made deep inroads as "bloodless scalpels." Recently, however, the US Food and Drug Administration (FDA) approved a treatment that takes advantage the specificity of certain photosensitive chemicals to treat a variety of maladies. Shown especially effective at treating early stage cancers, photodynamic therapy, or PDT as it is commonly known, uses light to activate a photosensitive drug that creates a toxic form of oxygen when exposed to light. Because the drug remains in cancerous tissue longer than in healthy tissue, the treatment can destroy tissue selectively. Although only one photosensitive drug has been approved by the FDA, Photofrin, more companies are lining up new photosensitizers. The treatment is especially appealing for cancer because it lacks many of the nasty side effects of traditional cancer therapies. Unlike chemotherapy or radiation treatment, PDT's worst side effect may be an increased sensitivity of the skin to sunlight. Unlike surgery, tumors destroyed with PDT usually leave no scars.