The best way to understand how drug addiction works is by considering that it can affect a person on both a mental and a physical level.
As the chemical compounds go to work they will breakdown the natural immunities within a person’s body, until these chemicals are in a position to replace them in their entirety. Where the human body will produce endorphins and natural chemicals to use on a daily basis; it will instead have to adapt and rely on the replacement chemicals that are being provided by the narcotics, alcohol, or both.
As our bodies adapt to these new toxins, they will begin to make use of them in place of the natural composites that a healthy person will possess. This can lead to a complete dependency on the consumption of a particular substance and it’s one of the main reasons why addicts need to be weaned off, as opposed to going ‘cold-turkey’, because their bodies can react negatively (and sometimes fatally) to being restricted of the chemicals that they now require.