The present invention relates to a nozzle for the injection of a liquid under pressure (3), comprising a vertical feed pipe (1) surmounted by a hollow head (3), the liquid under pressure being conducted between the outer wall (4) of the vertical feed pipe (1) and an inner tube (5), the upper part of the nozzle comprising at least one lateral orifice (6) for expelling the liquid under pressure (3). This nozzle is characterized in that the upper end of the inner tube (5) of the vertical feed pipe (1) emerges at a height above the lateral orifice(s) (6), thereby making it possible, when stopping the injection of the liquid under pressure (3), to introduce a gas under pressure (8) via the inner tube (5) of the vertical feed pipe (1). which generates an overpressure in the upper part of the nozzle in order, on the one hand, to lower the level of the liquid (9) under pressure (3) below the lateral orifice(s) (6) and, on the other hand, to prevent any backflow of solids, liquids and/or gases, from the medium into which the liquid under pressure (3) is injected. towards the inside of the nozzle.