While you grow and develop, your brain decides which biochemicals it needs to be healthy. The most important brain chemicals, called neurotransmitters, influence your mood, sleep, and appetite, among other things. If you take drugs and alcohol during your teens, your brain confuses these substances with natural neurotransmitters and incorrectly reads your level of development or maturity. Your brain unfortunately programs itself for wrong settings. You crave drugs or alcohol to correct these imbalances.