Nail biting (onychophagia) is an oral, unconscious, compulsive habit that can be found in both children and adults. Children normally grow out of nail biting when they reach their late teens or early 20s, but some people continue the habit because they have learnt to associate stress relief or comfort from it. Nail biting as a compulsive habit is where the nails are bitten past the nail bed, which causes the fingertips to become very sensitive to pain (usually at the place where the skin meets the edge of the nail). It also includes biting the skin surrounding the nails and the cuticles; which are often bitten until they bleed.
Because nail biting is a unconscious habit, it is automatic and no conscious thought is required to actually undertake the activity. Usually the person is involved in other activities, such as reading or watching TV.

Nail biting can also be a conscious habit where rough skin, broken parts of the nail and broken parts of the cuticle are bitten off to ensure the nails look and feel at their best.

In some cases, the damage to the nails can become so severe that the skin on the fingertips can become infected. This is due to constant contact with saliva which reddens the skin and infects it with bacteria. Bacteria like warm, moist conditions to grow and the skin by the nail edge is perfect for this to occur. This can leave the skin susceptible to microbial and viral infections. Biting the nails and skin that are infected can transport the infection to the mouth, which may result in dental problems such as gingival injury.

Hypnosis uses habit breakers, aversion therapy and suggestions to help prevent and stop the habit.