What does the Bible say about self-harm?

In his first letter to the church in Corinth, Paul writes (1 Cor 6:19-20):

Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God? You are not your own, for you were bought with a price. So glorify God in your body.

So we must consider our bodies as precious before God and not to be pointlessly damaged through substance abuse or deliberate mutilation.

On the other hand, Jesus says (Matthew 5:29-30)

If your right eye causes you to sin, tear it out and throw it away. For it is better that you lose one of your members than that your whole body be thrown into hell. And if your right hand causes you to sin, cut it off and throw it away. For it is better that you lose one of your members than that your whole body go into hell.

Of course Jesus did not mean to be taken literally. He said something like this on two occasions (see Matthew 18:8-9 for the other time) and both times it was in the context of things that tempt us to sin. In other words, Jesus meant “Take drastic action, if necessary, to get rid of the things that cause you to sin. It is better to endure pain and lose something close to you than to keep on sinning and miss out on eternal life.” The statement was intended to shock—to make it clear how important it is to resist temptation, even to the point of pain.