“Dual diagnosis” denotes the co-occurrence of a psychiatric condition, in our case schizophrenia, and a drug or alcohol use disorder. The term is neither precise (other dual diagnoses exist, for example, mental illness with developmental disorders) nor does it delineate a homogeneous class of patients (different mental disorders ranging from anxiety disorders to psychosis combined with any use to dependence), but the term has stuck. It came into being when, in the 1980s, a new cohort of “young adult chronic patients” who had never been institutionalized overwhelmed a treatment system that was ill-prepared to treat poorly compliant, drug-misusing patients with schizophrenia in the community, leading to the phenomenon of revolving-door psychiatric admissions.

In this chapter, I am discussing substance use in patients with diagnosed schizophrenia; the diagnostic difficulties that arise regarding drug-induced psychosis versus schizophrenia are dealt with in a separate chapter (Chapter 4). Smoking is so common and its consequences so devastating that I devote the whole next chapter to nicotine dependence. Caffeine, another common comorbidity, deserves a brief mention. In moderation, caffeine can be useful to counteract drug-induced sedation. However, excessive caffeine use can cause caffeine intoxication (“caffeinism”). Consider caffeinism in your restless patient with sleep problems.