A method and apparatus for rectal fistula laser treatment wherein an insertion tube of a coupler is inserted into a fistula from proximal to distal end. The insertion tube is then purged using sterile water and/or air. A flexible optical waveguide is then inserted through the coupler including the insertion tube until the end of the optical waveguide protrudes 1-3 mm from the distal end of the insertion tube. The optical waveguide is secured to the coupler by a lock nut of the coupler. The insertion tube locked to the optical waveguide is withdrawn through the fistula while CO2 laser radiation is passed down the optical waveguide so that laser radiation is delivered to the inner wall of the fistula at a specific rate as the laser radiation treats the fistula.
