When teenagers medicate, almost always what happens, either by experimentation or by accident, they discover that a drug actually makes them feel better.
The problem with self-medicating is, although teenagers who do it, regularly express symptom relief, almost always the drug is never the right drug for the situation. The thing about using drugs to self-medicate is, often times, the drug their using is the underlying cause of the issue they are trying to medicate for or at least a contributing factor to that situation.
As in the case of the teenagers that treat depression by smoking marijuana. When you smoke marijuana for your depression, after you do so, you're more depressed. You can see how that feedback loop would cause quick spiral out of control in the wrong person.
What can parents do to help teenagers around self-medicating issues? I think the most important thing they can do, is not ignore or minimize psychological symptoms in their children. Think about it for a second. The reason the child is self-medicating is because they have a problem. What you can do for your kid is this: Listen to their teachers. Listen to their coaches. Listen to their counselors, and talk to them.
What issues are you going through right now? What things are bothering you the most? Are you having problems that you would like to discuss with me? One of the things, when kids are having trouble, sometimes just a couple of sessions with a therapist or counselor will put them on the right path. Sometimes medication is required. When the right dose of the right medicine at the right time when a child is self-medicating might actually save them a lifetime of self-medicating, by doing the right thing at the right time.