An instrument for cutting body tissue has an instrument shaft, A blade, is adjacent to a distal end of the instrument shaft and can be folded out with respect to a longitudinal axis of the shaft and can be rotated about the longitudinal axis. An operating part is at a proximal end of the instrument shaft. A proximal end of the blade directed toward the operating part can be pivoted laterally out away from the longitudinal axis about an end of the blade that faces away from the proximal end and that is distally articulated at an articulation point. A tissue stop is arranged at the distal end of the instrument shaft. The tissue stop can be brought into an active position in which a stop surface facing the blade in the proximal direction has a specified fixed distance from the articulation point.