Nail biting

Nail biting is considered an impulse control disorder in the DSM-IV-R while the ICD-10 classifies it as "other specified behavioural and emotional disorders with onset usually occurring in childhood and adolescence".
Signs and symptoms
Bitten fin are broken skin on the cuticle. When they are improperly removed, they are susceptible to microbial and viral infections producing whitlows. Saliva may then redden and infect the skin. Finally it may also result in the transportation of bacteria that are buried under the surface of the nail, or pinworms from anus region to mouth. Nail biting is also related to dental problems, such as gingival injury and malocclusion of the anterior teeth. When the bitten-off nails are swallowed stomach problems can develop.
There are reported cases in the medical literature where after years of nail biting fingernails are severely deformed
Treatment
Behavioral treatments are based in discouraging the habit and replacing it with a more constructive habit. The most common treatment, as it is cheap and widely available, is a special clear nail polish that has to be applied to the nails. It releases a bitter flavor on contact with the mouth which discourages the habit and has demonstrated its effectiveness. There are also mouthpieces that prevent biting.[
Behavioral therapy is beneficial when simpler measures are not effective. Habit Reversal Training (HRT) seeks to "unlearn" the habit of nail biting and possibly replace it with a more constructive habit and has shown its effectiveness versus placebo both in children and adults.[9][10] In addition to HRT, stimulus control therapy is used to both identify and then eliminate the stimulus that frequently triggers biting urges.
Finally nail cosmetics can help to ameliorate nail biting social effects.
Children can wear footed pajamas as a reminder not to bite their toenails
Epidemiology
Nail biting affects around 30% of children between 7 to 10 years and 45% of teenagers.[1] The ten fingernails are usually equally bitten and approximately the same degree.[1] It may be underrecognized since individuals tend to deny or be ignorant of its negative consequences complicating its diagnosis.
Related disorders
Some related body-focused repetitive behaviors are dermatillomania (skin picking), dermatophagia (skin biting) or trichotillomania (urge to pull out hair). Nail biting appeared in a study to be more common in men with eating disorders than those without them. It is also more common among children and adolescents with obsessive–compulsive disorder.[16]