The light-sensitive drug (the photosensitizing agent) is injected into the bloodstream. It is taken up by the cells throughout the body. The photosensitizer gathers preferentially in cancer cells, but is not activated until exposed to laser light of the appropriate wavelength. During illumination the drug is triggered to interact with oxygen and form a transitory substance, known as singlet oxygen. This then destroys the cancer cells.

The laser light used in Photodynamic therapy is focused through a fibre-optic, and is shone for only a few minutes. The doctor holds the fibre-optic very close to the cancer so that the correct amount of light is delivered. This means that Photodynamic therapy causes the minimum amount of damage to normal, healthy cells.

In early-stage disease, the aim of treatment with Photodynamic therapy may be to completely remove and cure the cancer. In advanced disease, however, the goal may be to shrink the cancer in order to alleviate symptoms. In this scenario, Photodynamic therapy cannot cure the cancer.

Even patients who have had surgery, radiotherapy or chemotherapy in the past can be treated safely with Photodynamic therapy.

As with all treatments, patients may experience side effects following treatment with Photodynamic therapy. Following administration of the photosensitising agent, patients become highly sensitive to light. Precautions must be taken to avoid exposure of skin and eyes to direct sunlight or bright indoor lighting for a specified period following injection. Appropriate clothing and eyewear must be worn to prevent photosensitivity reactions. Other temporary side effects following Photodynamic therapy may include pain, which can be controlled with painkillers, swelling, bleeding and difficulty swallowing.

New, normal cells replace those cells killed by Photodynamic therapy allowing rapid healing to occur post-treatment. This avoids the scarring disfigurement that may occur with other types of tissue removal.