Many people are quick to say that certain exercises, like the behind-the-neck press, will cause injuries. Of course, anything can cause injuries, especially if you're doing anything worthwhile or rewarding. The trick is to make your body physically prepared to cope with the demands of training.
With the behind-the-neck overhead press, the risk simply comes down to your posture. Bad posture? It's not the best choice for you yet.
When do you do low reps and heavy weight? What about higher reps and light weight? Read this and finally feel good about writing your own training programs.