You might bite your lips casually when you're nervous, or you might compulsively chew them. Like blinking too often or picking your nails, it's probably something you unconsciously start doing when stress begins to build up, and you need an outlet. If you've gotten into the habit, however, you might bite enough to cause chapped lips, bleeding, or scarring, and you might feel unable to stop on your own. For mild cases, use a combination of simple habit-breaking techniques. For compulsive biting or chewing, visit a doctor and a therapist as soon as you can.

Breaking the Habit

Anticipate your biting. Notice when you bite, and reflect on how you feel. You might have the habit of biting your lips when you're feeling anxious, or bored. Tell yourself when you are about to enter a situation that might prompt biting, so you will be on the alert.

In many cases, biting is just one of several physical signs that you're feeling anxious. Other signs include shallow breathing, a faster heartbeat, blushing and sweating. When you feel one of these other symptoms, be ready to stop yourself from biting.
Use the competing response technique. When you feel the urge to bite, do something else that makes it impossible. Lick your lips, or rub a finger across them. Lightly bite a pencil or small cushion, or do something that makes it impossible to bite, such as breathing with your mouth open, speaking, or singing. This technique is used to treat deeply engrained compulsions, along with relaxation techniques and cognitive behavioral therapy.

Try this routine: when you want to bite, breathe deeply from your diaphragm for 60 seconds, relax your muscles one by one, then use your competing response for 60 seconds.
Substitute other movements, such as pursing your lips, chewing gum, whistling, or yawning. Remember not to touch your lips or face, as this can be unsanitary and cause later problems, such as pesky acne!
Ask your doctor or therapist for advice on developing your competing response technique. The same moves don't work for everyone.
Block your bites. Wearing an unpleasant-tasting lip balm can help remind you not to bite your lips. Try a medicated one meant to cure chapped lips or block the sun. If you are training yourself to lick instead of biting, try wearing a sweet-tasting lip balm. The taste and smell of the balm will help you think about licking your lips instead of biting. You can also apply the lip balm or lipstick when you have the urge to bite.

Rub a little mentholated lip conditioner right below your nose as a reminder.
If you are in a situation that triggers your lip biting, try sucking on hard candy, chewing gum, or wearing a mouth guard.
Visit your doctor. A doctor can help you isolate the cause of your biting, or can refer you to a specialist. Serious chewing and biting that causes bleeding, scarring or another damage to your lip or mouth requires medical attention. Lip biting is often a symptom of anxiety, but it can also be a symptom of obsessive-compulsive disorder (OCD), or body-focused repetitive behavior (BFRB).

Lip biting can be a tic. These are most common in young men and often disappear without treatment after a few months. A doctor can help you eliminate other possible causes
Ask your dentist about acquiring a mouth guard for help with lip biting, chewing, and clenching. If you tend to bite at night or during sedentary activities such as reading, watching television, or studying, these can be extremely helpful.
Seek the help of a therapist. Whether you bite your lips due to anxiety or a severe compulsion, getting therapy is more effective than medication. Your therapist will likely train you in habit reversal. This will involve mindfulness techniques, relaxation techniques, and competing response techniques. Ask for a referral for a therapist with experience treating compulsions and anxiety.

Ask your therapist about cognitive behavioral therapy, which focuses on the relationships between thoughts and behaviors.
Support groups can also be helpful, especially if you don't know other people who understand what you're going through.
Talk with a psychiatrist about anxiety medication. If nothing seems to be helping, you may have an anxiety disorder that could be mitigated with medication. Generalized anxiety disorder, obsessive-compulsive disorder, and other anxiety disorders don't always respond to talk therapy. Depending on your diagnosis and health profile, a psychiatrist might prescribe a medication to treat your anxiety.

This might include an antidepressant, such as medications in the selective serotonin reuptake inhibitor (SSRI) and serotonin-norepinephrine reuptake inhibitor (SNRI) drug class.
You might be prescribed Buspirone, a medication formulated specifically to treat anxiety, or Benzodiazepines, which are sedatives prescribed to treat severe cases.
Not all causes of lip biting respond to medication. BFRB, for example, responds best to habit reversal, with medications only prescribed for co-morbid conditions.