There is a high likelihood that most teens will be exposed to drugs and alcohol, also the there is a good chance that Teens as young as 13 have already tried drugs as strong as cocaine.
“Most teens don't start using drugs expecting to develop a substance abuse problem, and while most teens probably see their drug use as a casual way to have fun, there are negative effects that are a result of this use of drugs. The biggest consequence to casual drug use can be that it develops into a true addiction. Most addicts don’t recognize when they have crossed the line from casual use to addiction.” By Teen-Drug-abuse.com
Most teens don't think that they will become addicted, and simply use drugs or alcohol to have a good time and because of peer pressure. When teens become addicted they lose friends, develop health problems, start to fail in school and lose motivation
The signs to look for are declining interest in activities you once enjoyed, changes in school performance, and unpredictable mood swings that seem to be about more than just hormones.
Abuse of drugs and alcohol can also change friendships, as teens begin to move away from old friends who don't approve of their drug use and begin to associate with fellow drug users who will encourage and support one other's drug use. Parents should be very concerned when teens dump one group of friends for another.
Most teens who are addicted won't see a problem with their behavior or their drug use. Drugs make them feel good, and are a way to relieve the stress of school, problems at home, disagreements with friends, and other pressures of growing up.

The sooner you can recognize that you or your friend is abusing alcohol or other drugs, the sooner you can seek help. If you notice changes in behavior, changes in friends, lying about after school or weekend activities, changes in mood, or depression you might have a problem with substance abuse.
If you or someone you care about has a drug problem, talk to them about it and encourage them to get help.Rehab can help you build a strong recovery.
If you are concerned about your own drug use, parents are probably the last people you want to ask for help, but they can help you to find the treatment program that will support and guide you through recovery. If you are friend of someone who has a substance abuse problem, talk to them about their problem and encourage them to get help. The sooner you or someone gets help, the more likely they are to achieve successful recovery.