Therapists treating persons with substance use disorders need to have a basic knowledge of the psychiatric, medical, and psychosocial effects associated with the various patterns of substance use. An excellent overview of these effects is found in the substance-related disorders section of DSM-IV-TR (American Psychiatric Association 2000). These effects will not be reviewed in any detail here other than to say that they are numerous and vary according to drug class and route of administration and can be very serious. Notable among adverse psychiatric effects are suicidal or homicidal ideation and psychosis, especially in association with stimulant use. Serious medical complications include overdose, cardiac arrhythmias, cirrhosis, infections, accidents, and drug-related violence. Stimulants, opioids, high doses of sedatives including alcohol, drug injecting, and unprotected sex are particularly likely to result in one or more serious medical complications.