Dentist warns that biting your nails could ruin your teeth. Whether you’re bored or stressed, biting your fingernails can seem like the perfect band-aid. Psychology Today explains that nail biting is quite common habit in adults, with 20 to 30 percent of the population estimated to indulge in this practice. While the reason why people start often differs, one theory is that it helps them to regulate their emotions. However, the coping mechanism that almost always begins in childhood could spell bad news for your smile, according to an expert. An award-winning cosmetic dentist, who works at the Chelsea Dental Clinic in West London, took to his TikTok to reveal how the common practice can lead to tooth damage and bacteria exposure.