Psychoactive drugs act by altering communication among neurons. Drugs can facilitate or interfere with neurotransmitter actions to cause pleasure or reduce pain, effects that can result in addiction. Drug actions occur as neurotransmitter is released into the synapse, at postsynaptic receptors, or at other locations. Of the 80 known neurotransmitters, only a few, including dopamine and glutamate, are closely involved in the development of addiction. Neurons adapt to and counteract drug actions by increasing or decreasing neurotransmitter release, and by changing the density of receptors. This neuroadaptation produces tolerance, and is responsible for the withdrawal syndrome, with effects that are opposite to the initial drug action. Lipid-soluble drugs, and drugs administered via inhalation or intravenous injection, rapidly enter the brain and produce stronger effects than water-soluble drugs or those administered by other routes, such as by oral ingestion. Non-pharmacological factors – such as expectation and environment – also influence the effects of drug use and are important in the development of addiction, as well as in benign uses of psychoactive drugs

Addressing important problems and processes in the assessment of substance use disorders (SUDs), this chapter reviews the types of information necessary for a formal diagnosis of a DSM-5 SUD. In addition, the chapter discusses seven assessment areas that must be examined if a given case is to be well evaluated. These include clinical history, strength of dependence syndrome, impaired volitional control, craving, neuroadaptation, negative consequences and pathological patterns, and stage of change. The origin and research background of each of these constructs are briefly outlined, a favored assessment tool for each is reviewed in detail, and alternative measures are also suggested. The assessment of SUDs is a multidimensional, complex process for which many excellent tools have been, and are still being, developed.