Nail biting may actually be harmful to you beyond the emotional effects. For instance…
1. Disease-Causing Bacteria
Your nails are an ideal location for bacteria to thrive, and that includes potentially pathogenic bacteria like Salmonella and E. coli (which would love to call the underside of your nail tips home).
As you bite your nails, those bacteria easily transfer into your mouth and the rest of your body, where they may lead to infections. Your fingernails may actually be twice as dirty as your fingers,2 considering they’re difficult to keep clean, making this a prime point of transfer for infectious organisms.
Although I’m not aware of any research on this, it’s often suggested (anecdotally) that people who bite their nails have stronger immune systems, and therefore get sick less often, than those who do not.
One potential explanation for this is that nail biting may help introduce pathogens from your environment to your immune system, helping it to learn and build defenses, similar to what occurs when people eat their boogers.
2. Nail Infections
Nail biters are susceptible to paronychia, a skin infection that occurs around your nails. As you chew your nails, bacteria, yeast, and other microorganisms can enter through tiny tears or abrasions, leading to swelling, redness, and pus around your nail.
This painful condition may have to be drained surgically. Bacterial infections caused by nail biting are actually one of the most common nail problems, according to the American Academy of Dermatology (AAD).
3. Warts Due to HPV Infections
Warts on your fingers caused by human papillomavirus, or HPV, are common among chronic nail biters. (Here I’m referring to the types of HPV that cause warts on your hands, as opposed to those that lead to genital warts and, rarely, cervical cancer.) These warts can easily spread to your mouth and lips as you bite your nails.
4. Dental Problems
Nail biting can interfere with proper dental occlusion, or the manner in which your upper and lower teeth come together when you close your mouth.
Your teeth may shift out of their proper position, become misshapen, wear down prematurely, and become weakened if you bite your nails over time. The Academy of General Dentistry estimates that frequent nail biters may rack up $4,000 in additional dental bills over the course of their lifetime.
5. Impaired Quality of Life
A study published this year found that people who chronically bite their nails report significantly higher quality of life impairment than those who do not.
The level of impairment rises with time spent on nail biting, the number of involved fingernails and those who report visible nail abnormalities. Tension when trying to resist nail biting, suffering due to nail biting or nail-eating behavior also negatively influenced quality of life.