Drugs work on your brain by tapping into its communication system. They are chemicals which can permanently alter the way nerve cells send, receive and process information. Each drug works differently because it has a different chemical structure.
Drugs like marijuana and heroin have a chemical structure that imitates the structure of the neurotransmitters in your brain. They fool receptors, and can lock into them to activate the nerve cells. What makes them dangerous is that they don't work the same way that the natural neurotransmitters in your brain do, so they send abnormal messages through the brain.
Drugs like amphetamine can prevent the normal recycling of natural neurotransmitters or they may cause nerve cells to release unnaturally large amounts of neurotransmitters, which can lead to an exaggerated message sent to the brain. This ultimately wreaks havoc on the communication channels of your brain. The difference between natural neurotransmitter production and that caused by amphetamines is equal to a whisper versus someone screaming in your ear.
All drugs hijack the reward circuitry of your brain, causing abnormal and uncommonly large amounts of dopamine to flood your system. This may last longer than a natural release of dopamine and causes the high associated with drugs. While you may begin using drugs voluntarily, eventually the drugs alter your brain function. This impairs your ability to feel normal or to think clearly and rationally without drugs. All of this contributes to the compulsive drug use and drug-seeking behaviors that are common with teen drug addiction.