PDT involves the use of a drug that is triggered when activated by light. For some types of early skin cancer, the drug is applied as a cream which can be activated by a range of sources of red light. It is widely available and gives good cosmetic results. For internal cancers, the light source is usually a laser and the drug is given by mouth or injection. PDT involves the patient taking the drug, which accumulates in the cancerous area. A laser is then used to shine light on the cancer, which activates the drug and causes local tissue destruction. Both the medicine and the laser are basically harmless until combined.