Cutting and other self-injury in teen girls primary method for regulating their negative emotions, or for coping with them. As a parent, you need to first, gain an understanding of why many women self-injure. People who self injure usually have been living in high stress, high conflict or high abuse situations at earlier stages of their life; in which they feel completely helpless to control or change their circumstances, and from they cannot escape. Often, the self-injurer's parents were in constant fights or there was physical or sexual abuse occurring. So people self injure to: 1) escape or avoid negative emotions they cannot handle; 2) distract themselves from emotional pain; 3) validate that they are actually living breathing, bleeding individuals (because they experience what is called depersonalization at times); 4) escape feelings of self-hate or self-loathing; 5) distract perpetrators from the abuse because an injury must be attended to; 6) symbolically punish the perpetrator.
But the key to getting over the behavior is become a much better LISTENER as a parent and help the teen put into words strong, negative emotions that start to trigger the urge to self-injure. So you would start to talk to your daughter about her moods, frustrations, feelings about herself, etc. Parents are often oblivious to what is happening emotionally with their teens. Also, your daughter quite probably could benefit from talking to someone who is safe to confide in, trusted, etc., to experience a healthy emotional catharsis; and then find a constructive way to remedy any immediate situation that triggered the self-injurious behavior. Many people who self-injure feel it is an emotional 'addiction', that they literally are driven to self-injure because they feel emotionally too overwhelmed and trapped by their bad feelings.
Now, a lot of parent are not aware of some of the following issues regarding the causes and 'purposes' of self injury: 1) self-injury is NOT a suicide gesture. As I said above, most commonly, it is the outgrowth of one's frustrated attempts to regulate strong aversive emotions such as anger, depression, frustration. It is nearly always a sign that the person has (historically) lived in a very difficult, usually abusive home situation (e.g, parents are fighting constantly, there are serious parent-child conflicts, or physical/sexual abuse going on). 2) what self-injury really means is that the person has regularly felt extremely helpless and hopeless about how to cope with difficult emotional issues in his/her life. Teens who self injure are also at high risk for developing anorexia nervous or bulimia nervous, and substance abuse.
So I just shared with you my standard 'orientation' to self-injurious behavior. I'm quite confident that much of not all of it applies to your daughter, and you may not realize it. What do you think?