I’m not talking about genetic predisposition (albeit certainly a factor). When you take a physically addicting drug it can cause alterations of genetic expression. In other words, your body will uptick, or reduce, the number of various receptors within the nervous system. This same effect is what causes withdrawal. Some drugs are physically addicting because they cause physiological alterations in the nervous system’s reward complex.

Genetic predisposition and narcotic alteration of genetic expression are two different concepts.