A transnasal delivery method for treating or preventing Parkinsons disease using photobiomodulation. An optical system is provided including a light source and an optical fiber. The optical system is advanced through the nasal cavity until a distal end of the optical fiber is positioned inside the sphenoidal sinus. Then the light source is activated to irradiate substantia nigra brain tissue with an effective amount of light in the treatment or prevention of Parkinsons disease.
