Photodynamic therapy includes two stages. During the first stage the photosensitising substance is administered to the patient in one or more ways (topical, intravenous, orally), and is absorbed by the target cells.

The second stage includes the activation of the photosensitising agent in the presence of oxygen through a light of specific wavelength.

Because the photosensitising substance is absorbed more by cells which proliferate more the treatment is selective, minimizing the tissue damage on healthy tissue.