Say goodbye to the good old friends. In many cases, troubled teens go through friends like clothes, or leave established good friendships for new friends that are not heading in a positive direction. These are more outward signs of what is going on inside. The old saying "show me a man's friends, and I will show you the man" is still very appropriate today. A child's friends are often cited as a parent's number one concern. Unfortunately, it is also something the parent can do very little about. Your teen is going to pick their own friends. In a very dangerous situation, the only thing you may be able to do is remove your teen entirely from the environment.
The Academy of Pediatrics (in the journal Pediatrics, March 2005) indicates changes in friends may also signal drug use.