ABSTRACT
Each drug has one or more routes by which it can get into the body and then into the blood. And, each drug gets into the brain by way of the blood. The blood distributes the drug throughout the brain and body. Even though the drug molecules end up all over the brain and body, they can have an effect only on the neurons and other cells with receptors that recognize them. Nothing happens to cells without the appropriate receptors. So, the psychoactive effects of a particular drug depend on the locations of the neurons with receptors to recognize it and on which brain circuits these neurons are part of.
