The small cylindrical vials attached to his mouth each contain a small amount of a powerful anaesthetic. One assumes that these are replaceable.
When Batman fights him after returning to Gotham, a punch damages this portion of the mask, detaching some of the vials and causing Bane immense pain and sending him into a furious rage. Talia later fixes the mask by reattaching them and this returns him to his "normal" self.
The powerful anaesthetic in question doesn't exist in real life, its potency and concentration are meant to draw parallels with the Venom drug Bane so frequently abuses in the comics.
EDIT: The mask itself is also designed to be intimidating, projecting a symbol of fear, a common theme across all three Dark Knight movies. Christopher Nolan himself said that they drew inspiration from Darth Vader, who of course requires the breathing apparatus in his helmet in order to live, to design Bane's Mask. The Mask's overall practicality may suffer as a result of this.