A junction with a narrowing step or tapered section for use with a nebulizer is described. The step promotes homogeneous distribution of droplets when the nebulizer is activated and droplets are inhaled. The junction comprises an inlet (4), an outlet (5), an injection zone (3) a mixing chamber (13) and a step (18). A nebulizer (16) is attached to the injection zone (3) and vapour from the nebulizer is mixed in the mixing chamber before passing beyond the step (18) and through the constriction (7).
