The two primary reasons why people bite the inside of their cheeks are casual bites that accidentally go the wrong way, as when one is trying to talk and eat at the same time, and a more chronic biting that comes from poorly aligned teeth, either as a result of temporomandibular joint disorder or some other dental disorder. Poorly aligned teeth cause bites to go in the wrong direction and often result in biting not only the cheek, but also the lips and tongue. The nervous responses that some people have are the third reason.
Some people bite the insides of their cheeks due to a condition known as keratosis, a type of obsessive-compulsive disorder, according to GoodTherapy.org. The repetitive behavior often begins in childhood and can last into adulthood. It may be a compulsive reaction to anxiety and stress.
Another condition that may cause chewing of the inside of the cheeks out of compulsion is dermatophagia, according to Wikipedia. Sufferers additionally may chew on the skin around their joints and fingernails along with the inside of their mouths and lips, resulting in blisters. If the behavior continues, calluses may form where the biting occurs, the site notes.
Biting the insides of the cheeks causes problems over time. First of all, biting one's cheek leaves behind sores, and mouth sores are often quite painful. Second, having misaligned teeth, as with TMJ or other maladies, leads to teeth moving, severe headaches and other issues. According to the Huffington Post, some researchers have hypothesized a connection between mouth sores and oral cancer, but more studies on that subject are necessary to make a firm connection between the two.
To fix the problem, the best place to start is being more conscious about chewing, so that one builds a habit of chewing in the right direction. For those with TMJ or another cause of misalignment, having issue fixed often stop cheek-biting. If nervous habits are the cause of cheek-biting, chewing on gum often makes an adequate substitute.