481Substance-Related and
Addictive Disorders
The substance-related  disorders encompass 10 separate classes of drugs: alco-
hol; caffeine; cannabis; hallucinogens (with separate categories for phencyclidine [or sim-
ilarly acting arylcyclohexylamines] and ot her hallucinogens); inhalants; opioids;
sedatives, hypnotics, and anxiolytics; stimul ants (amphetamine-type substances, cocaine,
and other stimulants); tobacco ; and other (or unknown) subs tances. These 10 classes are
not fully distinct. All drugs that are taken in excess have in common direct activation of
the brain reward system, which is involved in  the reinforcement of behaviors and the pro-
duction of memories. They produce such an in tense activation of the reward system that
normal activities may be neglected. Inst ead of achieving reward system activation
through adaptive behaviors, drugs of abuse directly activate the reward pathways. The
pharmacological mechanisms by which each class  of drugs produces reward are different,
but the drugs typically activate the system and produce feelings of pleasure, often re-
ferred to as a “high.” Furthe rmore, individuals with lower levels of self-control, which
may reflect impairments of brain inhibitory mechanisms, may be particularly predisposed
to develop substance use disord ers, suggesting that the root s of substance use disorders
for some persons can be seen in behaviors long before the onset of actual substance use
itself.
In addition to the substance-related disorders, this chapter also includes gambling dis-
order, reflecting evidence that gambling be haviors activate reward systems similar to
those activated by drugs of abuse and produce some behavi oral symptoms that appear
comparable to those produced by the substa nce use disorders. Other excessive behavioral