A method for effecting an oral treatment of teeth and/or gums using an intra-oral device that has a mouthpiece in which is embedded a flexible circuit board and arrays of spaced apart lamps. The mouthpiece has a curvature. The lamps may be light emitting diodes that generate electromagnetic radiation, preferably in the white and blue light spectrum and the infrared and ultraviolet light spectrum. The arrays are positioned to expose the facial and lingual sides of the teeth and/or gums for effecting the treatment when the mouthpiece is positioned to fit upper and lower rows of teeth within accommodating recesses. The flexible circuit board is flexed to exhibit a curvature that follows a curvature of the mouthpiece. Treatments include whitening teeth, desensitizing teeth, and treating gums to prevent periodontal disease.