1. You're eating germs
Your nails pack a lot of dirt, which means there are germs there. And even if you wash your hands frequently, it's difficult to wash away all those germs. When you put your fingers in your mouth and chew on the nails, you are transferring those germs inside your body. And that significantly increases your chance of becoming ill.
2. You can get painful nail infections
You can also spread those germs from nail to nail, which increases your risk of paronychia, an infection of the nail. Symptoms of paronychia include a painful, red, swollen area around the nail, often at the cuticle or at the site of a hangnail or other injury. If the infection is bacterial, there may be pus-filled blisters at the site. Also, if you chew on nails that have warts, which are caused by a virus, and then chew on other nails, warts can spread to other areas.
3. Nail biting is bad for your teeth and your breath
It's not recommended to use your teeth as tools, apart from chewing your food. Regularly biting your nails can cause your teeth to shift out of place, which can require correctional braces or a retainer. Nail biting could also cause your teeth to break or could damage your tooth enamel. The germs could also potentially infect, or cause irritation, to your gums. Also, the bacteria that is on your fingers or nails can linger in your mouth and cause halitosis, or bad breath.
4. You have a higher risk of hangnails and ingrown nails
If you constantly bite at your nails, chances are you'll bite off a bit more than you expected, and when a piece of torn skin at the root of your nail appears, that's a hangnail. Hangnails are open sores that can easily become infected. The best way to avoid the painful sores is to prevent the hangnail from forming in the first place by moisturizing regularly--and not chewing on your fingers. Although most ingrown nails occur on the toenails, biting your fingernails can cause your nails to grow under your skin in your fingers too. Ingrown nails can cause pain and swelling and potentially lead to infection and require surgery.
5. You can eat toxins if you polish your nails
If you paint your nails and put them in your mouth, you increase the risk of toxic poisoning. Although regular nail polishes have plenty of toxins, gel polishes have chemicals that can be harmful when ingested. And while the low amounts of toxicity likely won't show any harmful effects right away, we don't yet understand the potential long-term consequences.