Drugs are chemicals. Different drugs, because of their chemical structures, can affect the body in different ways. In fact, some drugs can even change a person’s body and brain in ways that last long after the person has stopped taking drugs, maybe even permanently.
Depending on the drug, it can enter the human body in a number of ways, including injection, inhalation, and ingestion. The method of how it enters the body impacts on how the drug affects the person. For example: injection takes the drug directly into the blood stream, providing more immediate effects; while ingestion requires the drug to pass through the digestive system, delaying the effects.
Most abused drugs directly or indirectly target the brain’s reward system by flooding the circuit with dopamine. Dopamine is a neurotransmitter present in regions of the brain that regulate movement, emotion, cognition, motivation, and feelings of pleasure. When drugs enter the brain, they can actually change how the brain performs its jobs. These changes are what lead to compulsive drug use, the hallmark of addiction.