The heterogeneity of SUD/addictions is also partly due to the drugs’ differences in chemical classes, routes of administration, neurobiological systems they act on, metabolic pathways, and psychopharmacologic effects. Accordingly, SUD are classified into groups defined by chemical origins of the drug or its effects. This classification, however, does not resolve clinical heterogeneity, as the categorical substance-specific SUD diagnosis corresponds to hundreds of different combinations of symptoms (Vanyukov et al., 2003b). This vagueness of the diagnostic threshold creates vast variation within groups even with the same drug-specific diagnosis.
