Nervous habits are often more annoying to the people around you than to yourself, but some types of fidgeting and fussing can do real harm. Here, experts reveal the reasons why nail-biting, hair-twirling, and other seemingly harmless habits can be hazardous to your health.

You bite your nails
It's one thing if you nervously bite your nails only during scary movies, but when it becomes a regular habit, it can damage both your nails and the skin around them, said Dr. Michael Shapiro, a New York City-based dermatologist. Germs from the mouth get transferred to the skin, and vice versa.

"Bacteria under the nails may also be transferred to mouth, causing infections of the gums and throat," Shapiro said.

Painting your nails may discourage you from chewing. No dice? Try tape to break the habit.

You twirl and pull your hair
Twisting and twirling a piece of hair around your finger can lead to damage to the root over time, said Dr. Ariel Ostad, a dermatologist based in New York City.

"This can result in temporary or permanent areas of hair loss as well as infection," Ostad said.

Obsessive hair pulling may be a sign of a psychiatric impulse control condition called trichotillomania, which requires psychotherapy and medication.

You crack your neck
Twisting your head forcibly to one side releases gases built up in the the joints between vertebrae and creates a popping sound. Although this may feel good, repeatedly cracking your neck can make the surrounding ligaments hypermobile and more susceptible to injury, said Dr. Michael Gleiber, a board-certified orthopedic surgeon and affiliate assistant professor at Florida Atlantic University's Charles E. Schmidt College of Medicine in Boca Raton. 

In addition, this excessive motion on the facet joints themselves can cause wear within the joints and may result in arthritis over time. In rare cases neck cracking may trigger a stroke, Gleiber said.

You touch your face
Repeatedly touching your face or picking at acne can damage the top very thin microscopic layers of the skin, said Dr. Jessica Krant, board certified dermatologist and founder of Art of Dermatology and assistant clinical professor of dermatology at SUNY Downstate Medical Center in New York City.

"If you bleed, you may have just created a permanent scar," she said. "Do not pick at pimples or itchy areas. Treat them gently with topical creams and plenty of moisturizer."

You grind your teeth
Clenching and grinding your teeth (bruxism) when you're under stress can wreak havoc with your oral health. Grinding can cause teeth to crack or break, which may require repair with crowns or root canals. It can also result in damage to the jaw joint in the form of temporomandibular joint disorder (TMJ), said Justin Philipp, who has a dental practice in Chandler, Ariz.

"People clench or grind their teeth as a response to stress. However, most cases are a result of pathology such as misaligned or missing teeth and a 'bad bite,'" he said.

Treatments include orthodontics to improve the bite and even Botox injections in the muscles, which can reduce the amount of force and, therefore, the potential damage.

You suck on hard candies
Sucking on hard candies bathes your teeth in sugar, which can lead to cavities, says Philipp. Bacteria feed off the sugar, which creates a perfect environment for tooth decay. Chomping down on hard candy can also risk damaging teeth or dental restorations, said Jack Ringer, president of the American Academy of Cosmetic Dentistry.

"Sucking on candies in moderation is fine provided the candies are sugarless and low in acidity," Ringer said.

You lick or bite your lip
Nervously licking your lips exposes them to your mouth's digestive enzymes, said Dr. Whitney Bowe, a New York board-certified dermatologist.

"These enzymes chew away at the skin and can lead to dermatitis and cheilitis (inflammation), which make lips appear dry and cracked," she said.

Biting your lips when under stress can cause the development of fibromas, firm flesh colored growths, that may require surgical removal, says Coyle S. Connolly, MD, dermatologist and president of Connolly Dermatology in New Jersey. Relax in a healthier way with these expert-approved stress-busting solutions.

You gnaw on the inside of your cheek
Like biting your nails, cheek-chewing can also become a nervous habit.

"Often the inside of the cheek gets swollen and it then becomes easier to continue biting the same spot," says Ringer. "Even after it heals the habit continues." 

Over time this can result in chronic inflammation, possible bleeding, and scarring of the area.

You chew gum
All that snapping and popping does more than annoy your coworkers. It may also put you at risk for TMJ from overuse of jaw muscles, Philipp said. Sugarless gum presents a different set of problems, mainly digestive ailments. Sorbitol, an artificial sweetener, produces an unpleasant laxative effect when eaten in excess (18 to 20 sticks a day). Swallowing excess air while chewing also increases risk of a gassy stomach, according to the National Digestive Diseases Information Clearinghouse (NDDIC).

"It is usually easier to try to replace the habit with another one than it is to quit, so try something a healthier switch such as drinking water," Philipp said.

You nibble the ends of pencils and pens
Germs can lurk on the ends of pens so this habit can expose you to nasty pathogens including cold viruses, said Ted Myatt, director of research compliance at the University of Rhode Island.

"An infected person likely has the virus on his or her fingers and spreads it through pens as well as computer keyboards and telephones,” he said.

And aside from the embarrassment of ink on your mouth from an exploding pen, chewing on writing instruments can damage teeth and dental work as well as injure the soft tissue and gums inside the mouth, Ringer said.