Complex sentence: Environmental issues are one cause of drug use among teenagers, if you put a child through treatment and then put them back into the same environment they left there is a great chance that they will go back to their previous behavior.
Split into simpler sentences: Environmental issues are one cause of drug use among teenagers. If you place a child through treatment and then place them back into the same environment that they left, there is a great chance that they will go back to their previous behavior.