A mouthpiece that adjusts manually to accommodate a broad range of different size sets of upper and lower teeth in the mouth and yet seals the treatment area from oxygen exposure. The mouthpiece includes one or more light emitters and one or more heat generators. A series of parallel texture bands are provides to guide and direct the light from the light emitters to diffuse generally evenly onto teeth to be treated. The seal arises from an inner surface of the mouthpiece titling inwardly so that a seal bead seals in the vicinity of the gum above the teeth to be treated. Method for effecting an oral treatment such as whitening or desensitizing teeth or treating gum disease. For whitening, a whitening gel is applied to the user's teeth, the mouthpiece is then positioned in the user's mouth such that a bite surface is between upper and lower arches, a main body is between a front surface of the teeth and an inner surface of the user's lips, and a seal bead is against the user's gums. Then, the light emitter(s) and heat generator(s) are activated to effect the whitening while causing an increase in the temperature of the main body thereby enabling the user to mold the main body and conform its shape to their mouth.
