An overtube for use with an endoscope includes a tube body defining a longitudinal axis, a central channel defined in the tube body and extending along or parallel to the longitudinal axis between proximal and distal end portions of the tube body, and first and second side channels defined in the tube body and extending along or parallel to the longitudinal axis between the proximal and the distal end portions of the tube body. The central channel is sized and configured to receive an endoscope. The first and second side channels are circumferentially spaced part from each other and disposed between the central channel and an outer surface of the tube body. Each of the first and second side channels is sized and configured to receive a retraction device, and each has an elongated arcuate cross section or perimeter at the distal end portion of the tube body.