Abstract: A single-use annular mouthpiece for use with a diagnostic instrument such as an endoscope is provided. The mouthpiece protects the endoscope from damage and enhances patient comfort while giving a visible indication of any prior use. The mouthpiece includes a first relatively rigid portion having an annular main body shaped and dimensioned to be held in a patient's mouth and to provide clearance for an instrument through its central opening. The rigid portion includes an annular lip extending radially from the main body at one end and an annular flange extending radially from the main body at another end. The flange is shaped and dimensioned to overlie a patient's lips. The mouthpiece includes a second deformable compressible annular portion mounted around the annular main body of the first portion. The second portion is permanently deformable when subject to a patient'bite. The second portion is sized and shaped to substantially encircle the main body portion and to fit between the lip and the flange.