The invention relates to a mouthpiece (1) for a medical device (56). The mouthpiece extends along a first axis (2) from a rear portion (4) to a front portion (6) with a middle portion (8) arranged therebetween. The rear portion is for attachment to a medical device and the front portion is for inserting into the mouth of a user. The middle portion has a cross section parallel with a second axis (10) substantially perpendicular to the first axis. The cross section extends in a direction substantially parallel with a second axis from a first limit (12) to a second limit (14) and has a width (16) substantially perpendicular to both the first and second axes. The cross section includes a divergent portion (18) between the first limit and a widest limit (20), at which the width is a maximum, and a convergent portion (22) between the widest limit and the second limit. The widest limit is a first distance (24) from the first limit and a second distance (26) from the second limit and the first distance is greater than the second.
