Drugs such as heroin can influence how the brain functions by sending agonist molecules to the receptor cites of the cell. These agonist molecules imitate the effect of neurotransmitter molecules by filling the receptor sites of the cell to simulate the same effect of normal endorphins. This synthesizes the production of endorphins through an external influence instead of the natural methods such as exercise or pain. One way that heroin addiction starts, recalled by users as chasing the first high, begins with the first use. The level of euphoria achieved during someone’s first use of heroin will never be achieved again. This is because all of the body’s regular endorphin producing glands are functioning during the first use of heroin, after the first use the amount of endorphins the body naturally produces decreases. When that same person uses heroin a second time, they get a slightly less euphoric high, due to the decrease in production of natural endorphins. Sometimes to achieve the same high, users will up their intake of heroin to get go for their first high, which only decreases the body’s production of natural endorphins at an accelerated rate. Eventually if the drug is abused often enough, the body begins to rely solely on the agonist molecules of the external source completely replacing the body’s naturally produced endorphins. This is the stage of absolute addiction, where the body needs the drug to function naturally, and without, goes in withdrawal.
Exploring Psychology Seventh Edition, 2008, World Publishers