A partially blocked airway is often the cause of mouth breathing, usually due to allergies or enlarged adenoids or tonsils. A doctor should evaluate any of these conditions. Frequent mouth breathing can cause dry, red, swollen gums. This can be especially noticeable around erupting baby and permanent teeth.

In children younger than 8, about half do some breathing through their mouths, presumably not due to a medical problem. Most children outgrow this habit by the age of 8.¹

The relationship between ongoing (chronic) mouth breathing and malocclusion ("poor bite") is unclear, although the two are often seen together. The most common trait of people who chronically breathe through their mouths is an elongated (longer) lower face and a narrowed upper arch in the mouth (maxillary constriction). Cheek muscles pressing in on the upper side teeth cause these traits. However, experts still question whether mouth breathing is really responsible for these skeletal and dental changes.